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See O Senge et vi 


ste ee 


The person charging this material is re- 
sponsible for its return to the library from 
which it was withdrawn on or before the 
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L161—O-1096 


A SYSTEM | ae 


SYNTHETIC PHILOSOPHY. = 


THE PRINCIPLES 


OF 


fee) HO G Y:. 


EY 


HERBERT SPENCER, 
AUTHOR OF ‘‘SOCIAL STATICS,” ‘‘ THE FRINCIPLES OF PSYCHOLOGY,” 


“ESSAYS : SCIENTIFIC, POLITICAL, AND SPECULATIVE,” 
‘* FIRST PRINCIPLES,” ETC. 


VOL. IL 


STEREOTY PED—THIRD THOUSAND. 


WILLIAMS AND NORGATEH, 


14, HENRIETTA STREET, COVENT GARDEN, LONDON ; 
AND 20, SOUTH FREDERICK STREET, EDINBURGH. 


1880. 


The right of Translation is reserved. 


PREFACE TO VOL. LL 


SS RE 


THE proof sheets of this volume, like those of the last 
volume, have been looked through by Dr. Hooker and Prof. 
Huxley; and I have, as before, to thank them for their 
valuable criticisms, and for the trouble they have taken in 
checking the numerous statements of fact on which the argu- 
ments proceed. The consciousness that their many duties 
render time extremely precious to them, makes me feel how 
heavy is my obligation. 

Part IV., with which this volume commences, contains 
numerous figures. Nearly one half of them are repetitions, 
mostly altered in scale and simplified in execution, of figures, 
or parts of figures, contained in the works of various 
Botanists and Zoologists. Among the authors whom I have 
laid under contribution, | may name Berkeley, Carpenter, 
Cuvier, Green, Harvey, Hooker, Huxley, Milne- Edwards, 
Ralfs, Smith. The remaining figures, numbering 150, are 
from original sketches and diagrams. 

The successive instalments which compose this volume, 
were issued to the Subscribers at the following dates :—No. 
13 (pp. 1—80) in January, 1860; No. 14 (pp. 81—160) in 
June, 1865; No. 15 (pp. 161—240) in December, 1865; No. 
16 (pp. 241—820) in June, 1866; No. 17 (pp. 821—400) in 
November, 1866; and No. 18 (pp. 401—4566) in March, 1867. 


London, March 23rd, 1867. 


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CONTENTS OF VOL. IL. 


PART I[V.—MORPHOLOGICAL DEVELOPMENT. 
CHAP. 

I.—THE PROBLEMS OF MORPHOLOGY Hep 

II.—THE MORPHOLOGICAL COMPOSITION OF PLANTS sve 
I1I.—THE MORPHOLOGICAL COMPOSITION OF PLANTS, CON- 

TINUED ost te re ce 

IV.—THE MORPHOLOGICAL COMPOSITION OF ANIMALS .., 
V.—-THE MORPHOLOGICAL COMPOSITION OF ANIMALS, CON- 


TINUED vas wae aoe vee 
VI.——MORPHOLOGICAL DIFFERENTIATION IN PLANTS eee 
VII.—THE GENERAL SHAPES OF PLANTS eee soe 
VIII.—THE SHAPES OF BRANCHES eee eee eee 
IX.—THE SHAPES OF LEAVES see ees eee 
X.—THE SHAPES OF FLOWERS eee eee eee 
XI.—THE SHAPES OF VEGETAL CELLS eee oe 


XII.—CHANGES OF SHAPE OTHERWISE CAUSED ... me 
XIII.—MORPHOLOGICAL DIFFERENTIATION IN ANIMALS ase 
XIV.—THE GENERAL SHAPES OF ANIMALS “si oe 
XV.--THE SHAPES OF VERTEBRATE SKELETONS ... a 
XVI.—THE SHAPES OF ANIMAL CELLS ... vt ve 
XVII. SUMMARY OF MORPHOLOGICAL DEVELOPMENT ae 


PART V—PHYSIOLOGICAL DEVELOPMENT. 


I.—THE PROBLEMS OF PHYSIOLOGY... a 
Il.—DIFFERENTIATIONS BETWEEN THE OUTER AND INNER 


TISSUES OF PLANTS cee eee eve 
IlIL—DIFFERENTIATIONS AMONG THE OUTER TISSUES OF 


PLANTS eee eee eee eoe 
[7.—DIFFERENTIATIONS AMONG THE INNER TISSUES OF 


PLANTS tee eos oe ves 


113 
119 
130 
137 
146 
159 
162 
166 
169 
192 
210 
213 


221 


226 


vin SONTENTS. 


CHAP. * 
V.—-PHYSIOLOGICAL INTEGRATION IN, PLANTS ... eee 
VI.-—DIFFERENTIATIONS BETWEEN THE OUTER AND INNER 
TISSUES OF ANIMALS sac eee see 
VIIL—DIFFERENTIATIONS AMONG THE OUTER TISSUES OF 
ANIMALS foe 355 “se eee 
VIIIL—DIFFERENTIATIONS .AMONG THE INNER TISSUES OF 


ANIMALS ele ae eee eee 
IX.—PHYSIOLOGICAL INTEGRATION IN ANIMALS... coe 
X.—SUMMARY OF PHYSIOLOGICAL DEVELOPMENT vee 


PART VI.—LAWS OF MULTIPLICATION. 


I.—THE FACTORS sb oe eee eee 
rx PRIORI PRINCIPLE ... eee eee eee 
III.—OBVERSE A PRIORI PRINCIPLE ... =e eee 
IV.— DIFFICULTIES OF INDUCTIVE VERIFI{CATION see 
V.—ANTAGONISM BETWEEN GROWTH AND ASEXUAL GENESIS 
VI.—ANTAGONISM BETWEEN GROWTH AND SEXUAL GENESIS 
VII.—ANTAGONISM BETWEEN DEVELOPMENT AND GENESIS, 
ASEXUAL AND SEXUAL “ie coe ic 
VIII.-—ANTAGONISM BETWEEN EXPENDITURE AND GENESIS ... 
{x.—COINCIDENCE BETWEEN HIGH NUTRITION AND GENESIS 
X.—SPECIALITIES OF THESE RELATIONS eee tee 


XT.—INTERPRETATION AND QUALIFICATION see eee 


XII.—MULTIPLICATION OF THE HUMAN RACE ... eee 
XIII.—HUMAN POPULATION IN THE FUTURE a ees 
APPENDICES. 


A.—-SUBSTITUTION OF AXIAL FOR FOLIAR ORGANS IN PLANTS 
B.—A CRITICISM ON PROF. OWENS THEORY OF THE VER- 
TEBRATE SKELETON 


eee eee 


C.—ON CIRCULATION AND THE FORMATION OF WOOD IN 


PLANTS ve vee . 


a” 


PAGR 


275 


282 


291 


310 
365 
377 


391. 
397 
404 
412 
419 
428 


440) 
446 
454 
463 
470 
479 
494 


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sea a line oe * segeneral, ey ae neral. — 


PAE homogenous, £3 homogeneous. 
# Bailie ic ions, ,, distinctions. : < 
” the th ero ‘e the growth, 
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26, in, . into 


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TRACT lee es 


MORPHOLOGICAL DEVELOPMENT. 


aie, ls \ ae iy 


CHAPTER I. 


THE PROBLEMS OF MORPHOLOGY. 


§ 175. Tue division of Morphology from Physiology, is 
one which may be tolerably-well preserved, so long as we co 
not carry our inquiries beyond the empirical generalizations 
of their respective phenomena; but it is one which becomes 
in great measure nominal, when the phenomena are to be 
rationally interpreted. It would be possible, after analyzing 
our Solar System, to set down certain general truths respect- 
ing the sizes and distances of its primary and secondary 
members, omitting all mention of their motions ; and it would 
be possible to set down certain other general truths respect- 
ing their motions, without specifying their dimensions or 
positions, further than as greater or less, nearer or more re- 
mote. But on seeking to account for these general truths, 
arrived at by induction, we find ourselves obliged to con- 
sider simultaneously the relative sizes and places of the 
masses, and the relative amounts and directions of their 
motions. Similarly with organisms. Though we may frame 
sundry comprehensive propositions respecting the arrange- 
ments of their organs, considered as so many inert parts; and 
though we may establish several wide conclusions respecting 
the separate and combined actions of their organs, without 
knowing anything definite respecting the forms and positions 
of these organs; yet we cannot reach such a rationale of the 

1 * 


4. MORPHOLOGICAL DEVELOPMENT. 


facts as the hypothesis of Evolution aims at, without contem- 
plating structures and functions in their mutual relations. 
Everywhere structures in great measure determine functions ; 
and everywhere functions are incessantly modifying structures. 
In Nature, the two are inseparable co-operators ; and Science 
can give no true interpretation of Nature, without keeping 
their co-operation constantly in view. An account of organic 
evolution, in its more special aspects, must be essentially an 
account of the inter-actions of structures and functions, as 
perpetually altered by changes of conditions. 

Hence, when treating apart Morphological Development 
and Physiological Development, all we can do is to direct our 
attention mainly to the one or to the other, as the case may 
be. In dealing with the facts of structure, we shall consider 
the facts of function, only in such general way as is needful 
to explain the facts of structure ; and conversely when deal- 
ing with the facts of function. . 


§ 176. The problems of Morphology fall into two distinct 
classes, answering respectively to the two leading uzpects of 
Evolution. In things which evolve there go on two processes 
—increase of mass and increase of structure. Increase of 
mass is primary, and in simple evolution takes place almost 
alone. Increase of structure is secondary, accompanying or 
following increase of mass with more or less regularity, 
wherever evolution rises above that form which small inor- 
ganic bodies, such as crystals, present to us. The fundamental 
antagonism between Dissolution and Evolution consisting in 
this, that while the one is an integration of motion and dis- 
integration of matter, the other is an integration of matter 
and disintegration of motion ; and this integration of matter 
accompanying disintegration of motion, being a necessary 
antecedent to the differentiation of the matter so inte- 
grated ; it follows that questions concerning the mode in 
which the parts are united into a. whole, must be dealt with 


THE PROBLEMS OF MORPHOLOGY. 5 


before questions concerning the mode in which these parts 
become modified.* 

This is not obviously a morphological question. But an 
illustration or two will make it manifest, that fundamental 
differences may be produced between aggregates, by differences 
in the degrees of composition of the increments: the ultimate 
units of the increments being the same. Thus an accu- 
mulation of things of a given kind may be made by add- 
ing one at a time. Or the things may be tied up into 
bundles of ten, and the tens placed together. Or the tens may 
be united into hundreds, and a pile of hundreds formed. Such 
unlikenesses in the structures of masses, are habitually seen in 
our mercantile transactions. Articles which the consumer re- 
cognizes as single, the retailer keeps wrapped up in dozens, 
the wholesaler sends the gross, and the manufacturer supplies 
in packages of a hundred gross—that is, they severally increase 
their stocks by units of simple, of compound, and of doubly- 
compound kinds. Similarly result those differences of mor- 
phological composition which we have first to consider. An 
organism consists of units. These units may be aggregated 
into a mass by the addition of unit to unit. Or they may be 
united into groups, and the groups joined together. Or these 
groups of groups may be so combined as to form a doubly- 
compound aggregate. Hence there arise respecting each 
organic form, the question—is its composition of the first, 
second, third, or fourth order ?—does it exhibit units of a 
singly-compounded kind only; or are these consolidated into 
units of a doubly-compounded kind, or a triply-compounded 
kind? And if it displays double or triple composition, the 


* It seems needful here to say, that allusion is made in this paragraph to a pro- 
position respecting the ultimate natures of Evolution and Dissolution, which is 
contained in an essay on The Classification of the Sciences, published in March, 
1864. When the opportunity comes, I hope to make the definition there arrived 
at, the basis of a re-organization of the second part of First Principles : giving to 
that work a higher development, and a greater cohesion, than it at present pos- 
sesses, 


6. MORPHOLOGICAL DEVELOPMENT. 


homologies of its different parts become problems. Under - 
the disguises induced by the consolidation of primary, second- 
ary, and tertiary units, it has to be ascertained which answer 
to which, in their degrees of composition. 

Such questions are more intricate than they at first appear ; 
since, besides the obscurities caused by progressive integration, 
and those due to accompanying modifications of form, further 
obscurities result from the variable growths of units of the 
different orders. Just as an army may be augmented by re- 
cruiting in each company, without increasing the number of 
companies; or may be augmented by making up the full 
complement of companies in each regiment, while the 
number of regiments remains the same; or may be aug- 
mented by putting more regiments into each division, other 
things being unchanged; or may be augmented by adding to 
the number of its divisions without altering the components of 
each division ; or may be augmented by two or three of these 
processes at once; so, in organisms, increase of mass may be 
due to growth in units of the first order, or in those of the 
second order, or in those of still higher orders; or it may be 
due to simultaneous growth in units of several orders. 
And this last mode of integration being the general mode, 
puts difficulties in the way of analysis. Just as the structure 
of an army would be made less easy to understand, if com- 
panies often outgrew regiments, or regiments became larger 
than brigades ; so these questions of morphological composi- 
tion, are complicated by the indeterminate sizes of the units 
of each kind—relatively-simple units frequently becoming 
far more bulky than relatively-compound units. 


§ 177. The morphological problems of the second class, 
are those having for their subject-matfer the changes of shape 
that accompany changes of aggregation. The most general 
questions respecting the structure of an organism, having been 
answered when it is ascertained of what units it is composed as 
a whole, and in its several parts; there come the more special 


THE PROBLEMS OF MORPHOLOGY. 7 


questions concerning its form—form in the ordinary sense. 
After the contrasts caused by variations in the process of 
integration, we have to consider the contrasts caused by 
variations in the process of differentiation. To speak speci- 
fically—the shape of the organism as a whole, irrespect- 
ive of its composition, has to be accounted for. Reasons 
have to be found for the unlikeness between its general out- 
lines and the general outlines of allied organisms. And there 
have to be answered kindred inquiries respecting the propor- 
tions of its component parts :—Why, among such of these as 
are homologous with one another, have there arisen the 
differences that exist? And how have there been produced 
the contrasts between them and the homologous parts of or- 
ganisms of the same type? 

Very numerous are the heterogeneities of form that present 
themselves for interpretation under these heads. The ultimate 
morphological units combined in any group, may be differ- 
entiated individually, or collectively, or both: each of them 
may undergo changes of shape; or some of them may be 
changed and others not; or the group may be rendered mul- 
tiform by the greater growth of some of its units than of 
others. Similarly with the compound units, arising by union 
of these simple units. Aggregates of the second order may 
be made relatively complex in form, by inequalities in the 
rates of multiplication of their component units in diverse 
directions ; and among a number of such aggregates, numer- 
ous unlikenesses may be constituted by differences in their 
degrees of growth, and by differences in their modes of 
growth. Manifestly, at each higher stage of composition, the 
possible sources of divergence are multiplied still further. 

That facts of this order can be accounted for in detail, is 
not to be expected—the data are wanting. All that we may 
hope to do, is to ascertain their general laws. How this is to 
be attempted we will now consider. 


§ 178. The task before us is to trace throughout these 


8 MORPHOLOGICAL DEVELOPMENT. 


phenomena the process of evolution; and to show how, as 
displayed in them, it conforms to those first principles which 
evolution in general conforms to. Two sets of factors have 
to be taken into account. Let us look at them. 

The factors of the first class are those which tend directly 
to change an organic aggregate, in common with every other 
aggregate, from that more simple form which is not in equi- 
librium with incident forces, to that more complex form which 
is in equilibrium with them. We have to mark how, in corre- 
spondence with the universal law that the uniform lapses into 
the multiform, and the less multiform into the more multi- 
form, the parts of each organism are ever becoming further 
differentiated ; and we have to trace the varying relations to 
incident forces, by which further differentiations are entailed. 
We have to observe, too, how each primary modification of 
structure, induced by an altered distribution of forces, becomes 
a parent of secondary modifications—how, through the neces- 
sary multiplication of effects, change of form in one part brings 
about changes of form in other parts. And then we have also 
to note the metamorphoses constantly being induced by the 
process of segregation—by the gradual union of like parts 
exposed to like forces, and the gradual separation of like parts 
exposed to unlike forces. The factors of the second 
class which we have to keep in view throughout our interpret- 
ations, are the formative tendencies of organisms themselves 
—the proclivities inherited by them from antecedent organ- 
isms, and which past processes of evolution have bequeathed. 
We have seen it to be a necessary inference from various orders 
of facts (§§ 65, 84, 97,) that organisms are built up of certain 
highly-complex molecules, which we distinguished as physio- 
logical units—each kind of organism being built up of phy- 
siological units peculiar to itself. We found ourselves obliged 
to recognize in these physiological units, powers of arranging 
themselves into the forms of the organisms to which they be- 
long; analogous to the powers which the molecules of inor- 
ganic substances have of aggregating into specific crystalline 


THE PROBLEMS OF MORPHOLOGY. 9 


forms. We have consequently to regard this polarity of the 
physiological units, as producing, during the development of 
any organism, a combination of internal forces that expend 
themselves in working out a structure in equilibrium with 
the forces to which ancestral organisms were exposed; but 
not in equilibrium with the forces to which the existing organ- 
ism is exposed, if the environment has been changed. Hence 
the problem in all cases is, to ascertain the resultant of inter- 
nal organizing forces, tending to reproduce the ancestral form, 
and external modifying forces, tending to cause deviations from 
that form. Moreover, we have to take ‘:.c0 account, 
not only the characters of immediately-preceding ancestors, 
but also those of their ancestors, and ancestors of all degrees of 
remoteness. Setting out with rudimentary types, we have 
to consider how, in each successive stage of evolution, the 
structures acquired during previous stages, have been ob- 
scured by further integrations and further differentiations ; 
or, conversely, how the lineaments of primitive organisms 
have all along continued to manifest themselves under the 
superposed modifications. 


§ 179. Two ways of carrying on the inquiry suggest 
themselves. We may go through the several great groups 
of organisms, with the view of reaching, by comparison 
of parts, certain general truths respecting the homologies, 
the forms, and the relations of their parts; and then, having 
dealt with the phenomena inductively, may retrace our steps 
with the view of deductively interpreting the general truths 
reached. Or, instead of thus separating the two inves- 
tigations, we may carry them on hand in hand—first 
establishing each general truth empirically, and then pro- 
ceeding to the rationale of it. This last method will, I 
think, conduce to both brevity and clearness. Let us now 
thus deal with the first class of morphological problems. 


CHAPTER ILI. 


THE MORPHOLOGICAL COMPOSITION OF PLANTS, 


§ 180. Evolution implies insensible modifications and 
gradual transitions, which render definition difficult—which 
make it impossible to separate absolutely the phases of or- 
ganization from one another. And this indefiniteness of 
distinction, to be expected @ priort, we are compelled to re- 
cognize @ posteriori, the moment we begin to group morpho- 
logical phenomena into general propositions. Thus, on in- 
quiring what is the morphological unit, whether of plants or 
of animals, we find that the facts refuse to be included in any 
rigid formula. The doctrine that all organisms are built up 
of cells, or that cells are the elements out of which every 
tissue is developed, is but approximately true. ‘There are 
living forms of which cellular structure cannot be asserted ; 
and in living forms that are for the most part cellular, there 
are nevertheless certain portions which are not produced by 
the metamorphosis of cells. Supposing that clay were the only 
material available for building, the proposition that all houses 
are built of bricks, would bear about the same relation to the 
truth, as does the proposition that all organisms are composed 
of cells. This generalization respecting houses would be 
open to two criticisms: first, that certain houses of a primi- 
tive kind are formed, not out of bricks, but out of unmoulded 
clay ; and second, that though other houses consist mainly of 
bricks, yet their chimney-pots, drain-pipes, and ridge-tiles 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. Il 


do not result from combination or metamorphosis of bricks, 
but are made directly out of the original clay. And of like 
natures are the criticisms which must be passed on the 
generalization, that cells are the morphological units of or- 
ganisms. To continue the simile, the truth turns out to 
be, that the primitive clay or protoplasm out of which 
organisms are built, may be moulded either directly, or 
with various degrees of indirectness, into organic struc- 
tures. The physiological units which we are obliged to as- 
sume as the components of this protoplasm, must, as we have 
seen, be the possessors of those complex polarities which re- 
sult in the structural arrangements of the organism. The 
assumption of such structural arrangements may go on, and, 
in many cases, does go on, by the shortest route; without the 
passage through what we call metamorphoses. But where 
such structural arrangements are reached by a circuitous 
route, the first stage is the formation of these small aggre- 
gates, which, under the name of cells, are currently regarded 
as morphological units. 

The rationale of these truths appears to be furnished by the 
hypothesis of evolution. We set out with molecules one 
degree higher in complexity than those molecules of nitro- 
genous colloidal substance into which organic matter 1s 
resolvable ; and we regard these somewhat more complex mo- 
lecules as having the implied greater instability, greater sen- 
sitiveness to surrounding influences, and consequent greater 
mobility of form. Such being the primitive physiological 
units, organic evolution must begin with the formation of a 
minute aggregate of them—an aggregate showing vitality 
only by a higher degree of that readiness to change its form 
of aggregation, which colloidal matter in general displays ; 
and by its ability to unite the nitrogenous molecules it meets 
with, into complex molecules like those of which it is com- 
posed. Obviously, the earliest forms must have been minute ; 
since, in the absence of any but diffused organic matter, no 
form but a minute one could find nutriment. Obviously, too, 


12 MORPHOLOGICAL DEVELOPMENT. 


it must have been structureless; since, as differentiations are 
producible only by the unlike actions of incident forces, there 
could have been no differentiations before such forces had had 
time to work. Hence, distinctions of parts like those required 
to constitute a cell, were necessarily absent at first. And we 
need not therefore be surprised to find, as we do find, specks 
of protoplasm manifesting life, and yet showing no signs of 
organization. A further stage of evolution is 
reached, when thevery imperfectly integrated molecules form- 
ing one of these minute aggregates, become more coherent ; 
at the same time as they pass into a state of heterogeneity, 
gradually increasing in its definiteness. That is to say, we 
may look for the assumption by them, of some distinctions of 
parts, such as we find in cells, and in what are called uni- 
cellular organisms. They cannot retain their primordial uni- 
formity ; and while in a few cases they may depart from it 
but shghtly, they will, in the great majority of cases, acquire 
a very decided multiformity—there will result the compara- 
tively integrated and comparatively differentiated Protophyta 
and Protozoa. The production of minute aggregates 
of physiological units, being the first step ; and the passage of 
such minute aggregates into more consolidated and more 
complex forms, being the second step ; it must naturally hap- 
pen that all higher organic types, subsequently arising by 
further integrations and differentiations, will everywhere bear 
the impress of this earliest phase of evolution. From the 
law of heredity, considered as extending to the entire succes- 
sion of living things during the Earth’s past history, it 
follows, that since the formation of these small, simple organ- 
isms must have preceded the formation of larger and more 
complex organisms, the larger and more complex organisms 
must inherit their essential characters. We may anticipate 
that the multiplication and combination of these minute 
ageregates or cells, will be conspicuous in the early develop- 
mental stages of plants and animals; and that through- 
out all subsequent stages, cell-production and cell-differen- 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. Ig 


tiation will be dominant characteristics. The physiological 
units peculiar to each higher species, will, speaking generally, 
pass through this form of aggregation on their way towards 
the final arrangement they are to assume; because those 
primordial physiological units from which they are remotely 
descended, aggregated into this form. And yet, just as in 
other cases we found reasons for inferring (§ 131) that the 
traits of ancestral organization may, under certain conditions, 
be partially or wholly obliterated, and the ultimate structure 
assumed without passing through them; so, here, it is to be 
inferred that the process of cell-formation may, in some cases, 
be passed over. Thus the hypothesis of evolution 
prepares us for those two radical modifications of the cell- 
doctrine, which the facts oblige us to make. It leads us to 
expect that as structureless portions of protoplasm must have 
preceded cells in the process of general evolution ; so, in the 
special evolution of each higher organism, there will be 
an habitual production of cells out of structureless blastema. 
And it leads us to expect that though, generally, the physiolo- 
gical units composing a structureless blastema, will display 
their inherited proclivities by cell-development and meta- 
morphosis; there will nevertheless occur cases in which the 
tissue to be formed, is formed by direct transformation of the 
blastema. 

Interpreting the facts in this manner, we may recognize 
that large amount of truth which the cell-doctrine contains, 
without committing ourselves to the errors involved by the 
sweeping assertion of it. We are enabled to understand how 
it happens that organic structures are usually cellular in their 
composition, at the same time that they are not universally 
so. Weare shown that while we may properly continue to 
regard the cell as the morphological unit, we must constantly 
bear in mind that it is such only in a greatly-qualified sense.* 


* Let me here refer those who are interested in this question, to Prof. Hux- 


ley’s criticism on the cell-doctrine, published in the Medico-Chirurgical Review 
in 1853. 


14 MORPHOLOGICAL DEVELOPMENT. 


§ 181. These aggregates of the lowest order, each formed of 
physiological units united into a group that is structurally 
single, and cannot be divided without destruction of its 
individuality, may, as above implied, exist as independent 
organisms. The assumption to which we are committed by 
the hypothesis of evolution, that such so-called uni-cellular 
plants were at first the only kinds of plants, is in harmony 
with the fact that habitats not occupied by plants of higher 
orders, commonly contain these protophytes in great abund- 
ance and great variety. The various species of Protococcus, 
of Desmidiacee, and Diatomacee, supply examples of morpho- 
logical units living and propagating separately, under nu- 
merous modifications of form and structure. Figures 1, 2, 
and 3, represent a few of the commonest types. 


Mostly, simple plants are too small to be individually 
visible without the microscope. But, in some cases, these 
vegetal aggregates of the first order, grow to appreciable 
sizes. In the mycelium of some fungi, we have single cells 
developed into long branched filaments, or ramified tubules, 
that are of considerable lengths. An analogous structure 
characterizes certain tribes of Alge, of which Codium adherens, 
Fig. 4, may serve as an example. In Hydrogastrum, an- 
other alga, Fig. 5, we have a structure which is described as 

as 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 16 


simulating a perfect plant, with root, stem, bud, and fruit, 
all produced by the branching of a single cell. And 
among fungi, the genus Botrytis, Fig. 6, furnishes an illus- 
tration of allied kind. Here, though the size attained is 
much greater than that of many organisms which are mor- 
phologically compound, we are compelled to consider the 
morphological composition as simple; since the whole can no 
more be separated into minor wholes, than can the branched 
vascular system of an animal. In these cases, we have con- 
siderable bulk attained, not by a number of aggregates of 
the first order being united into an aggregate of the second 
order; but by the continuous growth of an aggregate of the 
first order. 


§ 182. The transition to higher forms begins in a very 
unobtrusive manner. Among these aggregates of the first 
order, an approach towards that union by which aggregates 
of the second order are produced, is indicated by mere juxta- 
position. Protophytes multiply rapidly; and their rapid 
multiplication sometimes causes crowding. When, instead 
of floating free in the water, they form a thin film on a moist 
surface, or are imbedded in a common matrix of mucus; the 
mechanical obstacles to dispersion result in a kind of feeble 
integration, vaguely shadowing forth a combined group. 
Somewhat more definite combination is shown us by such 
plants as Palmella botryoides. Here the members of a family 
of cells, arising by the spontaneous fission of a parent-cell, 
remain united by slender threads of that jelly-like substance 
which envelops their surfaces. In some Diatomacee, several 
individuals, instead of completely separating, hold together 
by their angles ; and in other Diatomacee, as the Bacillaria, 
a variable number of units cohere so slightly, that they are 
continually moving in relation to one another. 

This formation of aggregates of the second order, faintly 
indicated in feeble and variable unions like the above, may 
be traced through phases of increasing permanence and de- 


16 MORPHOLOGICAL DEVELOPMENT. 


finiteness, as well as increasing extent. In the yeast-plant, 
Fig. 7, we have cells which may exist singly, or joined into 
groups of several; and which have their shapes scarcely at 
all modified by their connexion. Among the Desmidiacee, it 
happens in many cases, that the two individuals produced by 
division of a parent-individual, part as soon as they are fully 
formed ; but in other cases, instead of parting they compose 
a group of two. Allied kinds show us how, by subsequent 
fissions of the adherent individuals and their progeny, there 
result longer groups ; and in some species, a continuous thread 


of them is thus produced. Figs. 8, 9, 10, 11, exhibit these 
20 


je) bibs: TOON i 


Desmidiacee ilustrate central aggregation; as shown in 
Figs. 12, 18, 14,15. Other instances of central aggrega- 
tion are furnished by such protophytes as the Gonium pector- 
ale, Fig. 16 (a being the front view, and 6 the edge view), 
and the Sarcina ventriculi, Fig. 17. Further, we have that 
spherical mode of aggregation of which the Volvox globator 
furnishes a familiar instance. 

Thus far, however, the individuality of the secondary ag- 
gregate is feebly pronounced: not simply in the sense that 
it is small; but also in the sense that the individualities of the 
primary aggregates are very little subordinated. But on 
seeking further, we find transitions towards forms in which 
the compound individuality is more dominant, while the sim- 
ple individualities are more obscured. Obscuration 
of one kind, accompanies mere increase of size in the second- 
ary aggregate: in proportion to the greater number of the 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 17 


morphological units held together in one mass, becomes their 
relative insignificance as individuals. We see this in the 
irregularly-spreading lichens that form patches on rocks ; 
and in such creeping fungi as grow in films or lamine ou 
decaying wood and the bark of trees. In these cases, how- 
ever, the integration of the component cells is of an almost 
mechanical kind. The aggregate of them is scarcely more 
individuated than a lump of inorganic matter: as witness the 
way in which the lichen extends its curved edges in this or 
that direction, as the surface favours; or the way in which 
the fungus grows round and imbeds the shoots and leaves that 
lie in its way, just as so much plastic clay might do. Though 
here, in the augmentation of mass, we see a progress towards 
the evolution of a higher type; we have as yet none of that 
definiteness required to constitute a compound unit, or true 
ageregate of the second order. Another kind of 
obscuration of the morphological units, is brought about by 
their more complete coalescence into the form of some struc- 
ture made by their union. This is well exemplified among 
the Conferve, and their allies. In Fig. 18, there are re- 


09 EE, 


presented the stages of a growing Mougeotia genuflexa, in 
which this merging of the simple individualities into the 
compound individuality, is shown in the history of a single 
plant ; and in Figs. 19, 20, 21, 22, 23, are represented a series 
of species from this group, and that of Cladophora, in which 
we see a progressing integration. While in the lower types, 


the primitive spheroidal forms of the cells are carcely 
VOL. II, 2 


18 MORPHOLOGICAL DEVELOPMENT. 


altered ; in the higher types, the cells are so fused together 
as to constitute cylinders divided by septa. Here, how- ° 
ever, the indefiniteness is still great: there are no specific 
limits to the length of any thread thus produced ; and none 
of that differentiation of parts required to give a decided in- 
dividuality to the whole. 

To constitute something like a true aggregate of the 
second order, capable of serving as a compound unit, that 
may be combined with others like itself into still higher 
aggregates, there must exist both mass and definiteness. 


§ 183. An approach towards plants which unite these cha- 
racters, may be traced in such forms as Bangia ciliaris, 
Fig. 24. The multiplication of cells here takes place, not in 
a longitudinal direction only, but also in a 
transverse direction; and the transverse 
multiplication being greater towards the 
middle of the frond, there results a differ- 
ence between the middle and the two ex- 
tremities—a character which, in a feeble 
&\ way, unites all the parts into a whole. Even 
ve this slight individuation is, however, very 
aH indefinitely marked; since, as shown by 
BS the figures, the lateral multiplication of cells 
‘ does not go on in a precise manner. 

%| From some such type as this there appear 
“to arise, by slight differences in the modes of 
growth, two closely-allied groups of plants, 
having individualities somewhat more pro- 

nounced. If, while the cells multiply lon- 
gitudinally, their lateral multiplication goes on in one direc- 
tion only, there results a flat surface, as in Ulva linza, Fig. © 
‘25 ; or where the lateral multiplication is less uniform in its 
rate, in types like Fig. 26. But where the lateral multiph- 
cation occurs in two directions transverse to one another, 
a hollow frond may be produced—sometimes irregulaaly 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 19 


spheroidal, and sometimes irregularly tubular ; as in Entero- 
morpha intestinalis, Fig. 27. And occasionally, as in Entero- 
morphacompressa, Fig. 28, thistubular frond becomes branched, 
Figs. 29 and 30 are magnified portions of such fronds ; show- 


ing the simple cellular aggregation which allies them with 
the preceding forms. 

In the common Fc of our coasts, other and somewhat 
higher stages of this integration are displayed. We have 
fronds preserving something like constant breadths; and 
dividing dichotomously with approximate regularity. Though 
the sub-divisions so produced are not to be regarded at all as 
separate fronds, but only as extensions of one frond, they 
foreshadow a higher degree of composition ; and by the com- 
paratively methodic way in which they are united, give to 
the aggregate a more definite, as well as a more complex, in- 
dividuality. Many of the higher lichens exhibit an 
analogous advance. While in the lowest lichens, the different 
parts of the thallus are held together only by being all 
-attached to the supporting surface, in the higher lichens the 
thallus is so far integrated that it can support itself by 
attachment to such surface at one point only. And then, in 
still more developed kinds, we find the thallus assuming a 
- dichotomously-branched form, and so gaining a more specific 
character as well as greater size. 

Q * 


20 MORPHOLOGICAT, DEVELOPMENT. 


Where, as in types like these, the morphological units 
show an inherent tendency to arrange themselves in a man- 
ner that is so far constant as to give characteristic propor- 
tions, we may say that there is a recognizable compound in- 
dividuality. Considering the Thallogens that grow in this 
way, apart from their kinships, and wholly with reference to 
their morphological composition, we might not inaptly de- 
scribe them as pseudo-foliar. 


§ 184. Another mode in which aggregation is so carried 
on as to produce a compound individuality of considerable 
definiteness, is variously displayed among other families of 
Alge. When the cells, instead of multiplying longitudin- 
ally alone, and instead of all multiplying laterally as well as 
longitudinally, multiply laterally only at particular places ; 
they produce a branched structure. 

Indications of this mode of aggregation occur among the 
Conferve and simple plants akin to them, as shown in Figs. 
22, 23. Though, in some of the more developed Alge which 
exhibit the ramified arrangement in a higher degree, the 
component cells are, like those of the lower Alg@, united to- 
gether end to end, in such way as but little to obscure their 
separate forms, as in Cladophora Hutchinsie, Fig. 31; they 


~ 
Y 


alee 
oe a. 
Ces 


nevertheless evince greater subordination to the whole of 
which they are parts, by arranging themselves more method- 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 21 


ically. Still further pronounced becomes the compound 
individuality, when, while the component cells of the 
branches unite completely into jointed cylinders, the com- 
ponent cells of the stem begin to multiply laterally, so as to 
form an axis distinguished by its relative thickness and com- 
plexity. Such types of structures are indicated by Figs. 32, 
33—figures representing small portions of plants which are 
quite tree-like in their entire outlines. On examining 
Figs. 34, 35, 36, which show the structures of the stems in 
these types, it will be seen, too, that the component cells in 
becoming more coherent, have undergone changes of form 
which obscure their individualities more than before: not 
only are they much elongated, but they are so compressed as 
to be prismatic rather than cylindrical. This structure, be- 
sides displaying integration of the morphological units car- 
ried on in two directions instead of one; and besides displaying 
this higher integration in the greater merging of the indi- 
vidualities of the morphological units in the general individu- 
ality ; also displays it in the more pronounced subordination 
of the branches and branchlets to the main stem. This differ- 
entiation and consolidation of the stem, brings all the second- 
ary growths into more marked dependence; and so renders 
the individuality of the aggregate more decided. 

We might not inappropriately call this type of structure 
pseud-axial. It simulates that of the higher plants in cer- 
tain leading characters. We see in it a primary axis along 
which development may continue indfienitely, and from 
which there bud out, laterally, secondary axes of like na- 
ture, bearing like tertiary axes; and this is the mode of 
erowth with which Phznogams make us familiar. But the 
resemblance goes no further ; for these pseud-axes are devoid 
of those lateral appendages—those leaves or foliar organs— 
which true axes bear, and the bearing of which ordinarily 
constitutes them true axes. 


§ 185. Some of the larger Alge supply examples of an 


22 MORPHOLOGICAL DEVELOPMENT. 


integration still more advanced: not simply inasmuch as 
they unite much greater numbers of morphological units 
into continuous masses; but also inasmuch as they com- 
bine the. pseudo-foliar structure with the pseud-axial struc- 
ture. Our own shores furnish an instance of this in the 
common Laminaria; and certain gigantic Fuct of the 
Antartic seas, supply yet better instances. In some of 
these, the germ develops a very long slender stem, which 
eventually expands into a large bladder-like or cylindrical 
air-vessel; and from the surface of this there grow out 
numerous leaf-shaped expansions. Another kind, Lessonia 
fuscescens, Fig. 37, shows us a massive stem growing up 
(an yA through water many feet deep—a stem which, 

} ‘N bifurcating as it approaches the surface, flat- 

i ie 


i : «ee f 
i Kami, tens out the ends of its subdivisions into fronds 
ti. Wy. 


fish 


ii aN like ribands. These, however, are not true 


foliar appendages, since they are merely ex- 
panded continuations of the stem. The whole 
plant, great as is its size, and made up though 
it seems to be of many groups of mor- 
phological units, united into a compound 
group by their marked subordination to a 
connecting mass, is nevertheless a single 
thallus. The aggregate is still an aggregate 
of the second order. 

But among certain of the highest A/ge, we do find some- 
thing more than this union of the pseud-axial with the 
pseudo-foliar structure. In addition to pseud-axes of 
comparative complexity; and in addition to pseudo-folia 
that are like leaves, not only in their general shapes, but 
in having mid-ribs and even veins; there are the be- 
ginnings of a higher stage of integration. Figs. 38, 39, 
and 40, show some of the steps. In Rhodymenia palmata, 
Fig. 38, the parent-frond is comparatively irregular in shape, 
and without a mid-rib; and along with this very imperfect 
integration, we see that the secondary fronds growing from 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 23 


PLATA LT. 


L 


NUS 
Rael, V 


As 


\\ ie 
38 f— 
ES i {Es 
tht \ ; 
39 LO 


the edges are distributed very much at random, and are by 
no means specific in their shapes. A considerable advance is 
displayed by Phyllophora rubens, Fig. 39. Here the frond, 
primary, secondary, or tertiary, betrays some approach to- 
wards regularity in both form and size; by which, as also by 
its partially-developed mid-rib, there is established a more 
marked individuality; and at the same time, the growth of 
the secondary fronds no longer occurs anywhere on the edge, 
in the same plane as the parent frond, but from the surface 
at specific places. Delesseria sanguinea, Fig. 40, illustrates 
a much more definite arrangement of the same kind. The 
fronds of this plant, quite regularly shaped, have their parts 
decidedly subordinated to the whole; and from their mid- 
ribs grow other fronds, which are just like them. ach of 
these fronds is an organized group of those morphological 
units which we distinguish as aggregates of the first order. 
And in this case, two or more such aggregates of the second 
order, well individuated by their forms and structures, are 
united together; and the plant composed of them is thus 
rendered, in so far, an aggregate of the third order. 

Just noting that in certain of the most-developed Alga, as 


24 MORPHOLOGICAL DEVELOPMENT. 


the Sargassum, or common gulf-weed, this tertiary degree of 
composition is far more completely displayed, so as to pro- 
duce among Thallogens a type of structure closely simulating 
that of the higher plants, let us now pass to the considera- 
tion of these higher plants. 


§ 186. Having the surface of the soil for a support and the 
air for a medium, terrestrial plants are mechanically circum- 
stanced in a manner widely different from that in which 
aquatic plants are circumstanced. Instead of being buoyed 
up by a surrounding fluid of specific gravity equal to their 
own, they have to erect themselves into a rare fluid which 
yields no appreciable support. Further, they are dis- 
similarly conditioned in having two sources of nutriment in 
place of one. Unlhke the Alga, which derive all the mate- 
rials for their tissues from the water bathing their entire 
surfaces, and use their roots only for attachment ; most of the 
plants which cover the EHarth’s surface, absorb part of their 
food through their imbedded roots: and part through their 
exposed leaves. ‘These two marked unlikenesses in the rela- 
tions to surrounding conditions, profoundly affect the respec- 
tive modes of growth. We must duly bear them in mind 
while studying the further advance of composition. 

The class of plants to which we now turn—that of Acrogens 
—is nearly related by its lower members to the classes above 
dealt with : so much so, that some of the inferior liverworts 
are quite licheniform, and are often mistaken for lichens. 
Passing over these, let us recommence our analysis with such 
members of the class as repeat those indications of progress 
towards a higher composition, which we have just observed 
among the more-developed Alge. The Jungermanniacee 
furnish us with a series of types, clearly indicating the transi- 
tion from an aggregate of the second order to an aggregate 
of the third order. F igs. 41, and 42, indicate the structure 
among the lowest of this group. Here there is but an incom- 
plete development of the second order of aggregate. The 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 29 


frond grows as irregularly as the thallus of a lichen: it is in- 
definite in size and outline, spreading hither or thither as the 
conditions favour. Moreover, it lacks the differentiations re- 


quired to subordinate its parts to the whole: it is uniformly 
cellular, having neither mid-rib nor veins; and it puts out 
rootlets indifferently from all parts of its under-surface. In 
Big 43, Jungermannia epiphylla, we have an advance on this 
type. There is here, as shown in the transverse section, Fig. 
44, a thickening of the frond along its central portion, pro- 
ducing something like an approach towards a mid-rib; and 
from this the rootlets are chiefly given off. The outline, too, 
is much less irregular; whence results greater distinctness 
of the individuality. A further step is displayed in Junger- 
mannia furcata, Fig. 45. The frond of this plant, compara- 
tively well integrated by the distribution of its substance 
around a decided mid-rib, and by its comparatively-definite 
outlines, produces secondary fronds—there is what is called 
proliferous growth ; and, occasionally, as shown in Fig. 46, 
representing an enlarged portion, the growth is doubly-pro- 
liferous. In these cases, however, the tertiary aggregate, so 
far as it is formed, is but very feebly integrated ; and its in- 
tegration is but temporary. For not only do these younger 
fronds that bud out from the mid-ribs of older fronds, develop 
rootlets of their own ; but as soon as they are well grown and 
adequately rooted, they dissolve their connexions with the 
parent-fronds, and become quite independent. From 
these transitional forms we pass, in the higher Jungerman- 
niacee, to forms composed of many fronds that are perman- 
ently united by a continuous stem. A more-developed ag- 


26 MORPHOLOGICAL DEVELOPMENT. 


gregate of the third order is thus produced. But though, 
along with increased definiteness in the secondary aggregates, 
there is here an integration of them so extensive and so re- 
gular, that they are visibly subordinated to the whole they 
form; yet the subordination is really very incomplete. In 
some instances, as in J. complanata, Fig. 47, the leaflets de- 
velop roots from their under surfaces, just as the primitive 
frond does; and in the majority of the group, as in Jd. 
capitata, Fig. 48, roots are given off all along the connecting 
stem, at the spots where the leaflets or frondlets join it: the 
result being, that though the connected frondlets form a 
physical whole, they do not form, in any decided manner, 
a physiological whole; since successive portions of the 
united series, carry on their functions independently of the 
rest. Finally, the most developed members of the 
group, present us with tertiary aggregates that are physio- 
logically as well as physically integrated. Not lying prone 
like the kinds thus far described, but growing erect, the stem 
and attached leaflets become dependent upon a single root or 
group of roots ; and being so prevented from carrying on their 
functions separately, are made members of a compound indi- 
vidual—there arises a definitely-established aggregate of the 
third degree of composition. 

The facts as arranged in the above order, are suggestive. 
Minute aggregates, or cells, the grouping of which we traced 
in § 182, showed us analogous phases of indefinite union, 
which appeared to lead the way towards definite union. We 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 27 


see here among compound aggregates, as we saw there 
among simple aggregates, the establishment of a specific 
form, and a size that falls within moderate limits of varia- 
tion. This passage from less definite extension to more de- 
finite extension, seems in the one case, as the other, to be ac- 
companied by the result, that growth exceeding a certain 
rate, ends in the formation of a new aggregate, rather than 
an enlargement of the old. And on the higher stage, as on 
the lower, this process, irregularly carried out in the simpler 
types, produces in them unions that are but temporary ; while 
in the more-developed types, it proceeds in a systematic way, 
and ends in the production of a permanent aggregate that is 
doubly compound. 

Must we then conclude, that as cells, or morphological 
units, are integrated into a unit of a higher order, which we 
call a thallus or frond; so, by the integration of fronds, there 
is evolved a structure such as the above-delineated species 
possess ? Whether this is the interpretation to be given of 
these plants, we shall best see when considering whether it is 
the interpretation to be given of plants that rank above them. 
Thus far we have dealt only with the Cryptogamia. We 
have now to deal with the Phanerogamia or Pheenogamia. 


CHAPTER III. 


THE MORPHOLOGICAL COMPOSITION OF PLANTS, 
CONTINUED. 


-§ 187. Tsar advanced composition arrived at im the 
Acrogens, is carried still further in the Endogens and Exo- 
gens. In these most-elevated vegetal forms, aggregation 
of the third order is always distinctly displayed; and aggre- 
gates of the fourth, fifth, sixth, &c., orders are very common. 

Our inquiry into the morphology of these flowering 
plants, may be advantageously commenced by studying the 
development of simple leaves into compound leaves. It is 
easy to trace the transition, as well as the conditions under 
which it occurs; and tracing it will prepare us for under- 
standing how, and when, metamorphoses still greater in de- 
gree, take place. 


§ 188. If we examine a branch of the common bramble, 
when in flower or afterwards, we shall not unfrequently find 
a simple or undivided leaf, at the insertion of one of the 
lateral flower-bearing axes, composing the terminal cluster of 
flowers. Sometimes this leaf is partially lobed ; sometimes 
cleft into three small leaflets. Lower down on the shoot, if 
it be a lateral one, occur larger leaves, composed of three 
leaflets; and in some of these, two of the leaflets may be 
lobed more or less deeply. On the main stem, the leaves, 
usually still larger, will be found to have five leaflets. Sup- 


fHE MORPHOLOGICAL COMPOSITION OF PLANTS. 29 


posing the plant to be a well-grown one, it will furnish all 
gradations between the simple, very small leaf, and the large 
composite leaf, containing sometimes even seven leaflets. 
Figs. 50 to 64, represent leading stages of the transition. 


d- 25 


/ 


What determines this transition ? Observation shows that the 
quintuple leaves occur where the materials for growth are 
supplied in greatest abundance; that the leaves become less 


30 MORPHOLOGICAL DEVELOPMENT. 


and less compound, in proportion to their remoteness from the 
main currents of sap; and that where an entire absence of 
divisions or lobes is observed, it is on leaves within the 
flower-bunch: at the ‘place, that is, where the forces that 
cause growth are nearly equilibrated by the forces that 
oppose growth ; and where, as a consequence, gamogenesis 18 
about to set in (§ 78). Additional evidence that the degree 
of nutrition determines the degree of composition of the leaf, 
is furnished by the relative sizes of the leaves. Not only, on 
the average, is the quintuple leaf much larger in its total area 
than the triple leaf; but the component leaflets of the one, are 
usually much larger than those of the other. The like con- 
trasts are still more marked between triple leaves and simple 
leaves. This connexion of decreasing size with decreasing 
composition, is conspicuous in the series of figures: the differ- 
ences shown, being not nearly so great as may be frequently 
observed. Confirmation may be drawn from the fact, that 
when the leading shoot is broken or arrested in its growth, 
the shoots it gives off (provided they are given off after the 
injury), and into which its checked currents of sap are thrown, 
produce leaves of five leaflets, where ordinarily leaves of three 
leaflets occur. Of course incidental circumstances, as varia- 
tions in the amounts of sunshine, or of rain, or of matter sup- 
plied to the roots, are ever producing changes in the state of 
the plant as a whole; and by thus affecting the nutrition of its 
leafsbuds at the times of their formation, cause irregularities 
in the relations of size and composition above described. But 
taking these causes into account, it is abundantly manifest 
that a leaf-bud of the bramble, will develop into a simple 
leaf or into a leaf compounded in different degrees, according 
to the quantity of assimilable matter brought to it at the 
time when the rudiments of its structure are being fixed. 
And on studying the habits of other plants—on observing 
how annuals that have compound leaves, usually bear simple 
leaves at the outset, when the assimilating surface is but 
small; and how, when compound-leaved plants in full growth 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 38l 


bear simple leaves in the midst of compound ones, the rela- 
tive smallness of such simple leaves shows that the buds from 
which they arose were ill-supplied with sap; it will cease to 
be doubted that a foliar organ may be metamorphosed into a 
group of foliar organs, if furnished, at the right time, with 
a quantity of matter greater than can be readily organized 
round a single centre of growth. An examination of the 
transitions through which a compound leaf passes into a 
doubly-compound leaf, as seen in the various intermediate 
forms of leaflets in Fig. 65, will further enforce this 
conclusion. 


KNW 


Here we may advantageously note, too, how in such cases, 
the leaf-stalk undergoes concomitant changes of structure. 
In the bramble-leaves above described, it becomes compound 
simultaneously with the leaf—the veins become mid-ribs while 
the mid-ribs become petioles. Moreover, the secondary stalks, 
and still more the main stalks, bear thorns similar in their 
shapes, and approaching in their sizes, to those on the stem ; 


o2 MORPHOLOGICAL DEVELOPMENT. 


besides simulating the stem in colour and texture. In the 
petioles of large compound leaves, like those of the com- 
mon Heraclewm, we still more distinctly see both mternal 
and external approximations in character to axes. Nor are 
there wanting plants whose large, though simple, leaves, are 
held out far from the stems, by foot-stalks that are, near the 
ends, sometimes so like axes, that the transverse sections of 
the two are indistinguishable; as instance the Calla Hthiopica. 

One other fact respecting the modifications which leaves 
undergo, should be set down. Not only may leaf-stalks as- 
sume to a great degree the characters of stems, when they 
have to discharge the functions of stems, by supporting many 
leaves or very large leaves; but they may assume the cha- 
racters of leaves, when they have to undertake the functions 
of leaves. The Australian Acacias furnish a remarkable 
illustration of this. Acacias elsewhere found, bear pinnate 
leaves ; but the majority of those found in Australia, bear what 
appear to be simple leaves. It turns out, however, that these 
are merely leaf-stalks flattened out into foliar shapes: the 
laminze of the leaves being undeveloped. And the proof 
is, that in young plants, showing their kinships by their em- 
bryonic characters, these leaf-like petioles bear true leaflets at 
their ends. A metamorphosis of like kind occurs in Ovalis 
bupleurifolia, Fig. 66. The fact most deserving of notice, 
however, is, that these leaf- 
stalks, in usurping the gene- 
ral aspects and functions of 
leaves, have also usurped their 
structures: though their ve- 
nation is not like that of the leaves they replace, yet they 
have veins, and in some cases mid-ribs. 

Reduced to their most general expression, the truths 
above shadowed forth are these :—That group of morphologi- 
cal units, or cells, which we see integrated into the compound. 
unit called a leaf, has, in each higher plant, a typical form; due 
to the special arrangement of these cells around a mid-rib and 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 33 


veins. Ifthe multiplication of morphological units, at the 
time when the leaf-bud is taking on its main outlines, exceeds 
a certain limit, these units begin to arrange themselves round 
secondary centres, or lines of growth, in such ways as to re- 
peat, in part or wholly, the typical form: the larger veins 
become transformed into imperfect mid-ribs of partially inde- 
pendent leaves; or into complete mid-ribs of quite separate 
leaves. And as there goes on this transition from a single 
agoregate of cells to a group of such aggregates, there simul- 
taneously arises, by similarly-insensible steps, a distinct 
structure which supports the several aggregates thus pro- 
duced, and unites them into a compound aggregate. These 
phenomena should be carefully studied; since they give us a 
key to more involved phenomena. 


§ 189. Thus far we have dealt with leaves ordinarily so 
called: briefly indicating the homologies between the parts of 
the simple and the compound. Let us now turn to the homo- 
logies among foliar organs in general. These have been 
made familiar to readers of natural history, by popularized 
outlines of “The Metamorphosis of Plants ’’—a title, by the 
way, which is far too extensive ; since the phenomena treated 
of under it, form but a small portion of those it properly in- 
cludes. 

Passing over certain vague anticipations that have been 
quoted from ancient writers, and noting only that some 
clearer recognitions were reached by Joachim Jung, a Ham- 
burg professor, in the middle of the 17th century ; we come 
to the Theorta Generationis, which Wolff published in 1759, 
andin which he gives a definite form to the conceptions that 
haye since become current. Specifying the views of Wolff, 
Dr Masters writes,—‘ After speaking of the homologous 
nature of the leaves, the sepals and petals, an homology 
consequent on their similarity of structure and identity of 
origin, he goes on to state that the ‘pericarp is manifestly 


composed of several leaves, as in the calyx, with this differ- 
VOL. II. 3 


34 MORPHOLOGICAL DEVELOPMENT. 


ence only, that the leaves which are merely placed in close 
contact in the calyx, are here united together ;’ a view which 
he corroborates by referring to the manner in which many 
capsules open and separate ‘into their leaves.’ The seeds, too, 
he looks upon as consisting of leaves in close combination. His 
reasons for considering the petals and stamens as homologous 
with leaves, are based upon the same facts as those which led 
Linnzeus, and, many years afterwards, Goethe, to the same 
conclusion. ‘In a word,’ says Wolff, ‘we see nothing in 
the whole plant, whose parts at first sight differ so remark- 
ably from each other, but leaves and stem, to which latter 
the root is referrible.’”’ It appears that Wolff, too, enunci-» 
ated the now-accepted interpretation of compound fruits: 
basing it on the same evidence as that since assigned. In 
the essay of Goethe, published thirty years after, these rela- 
tions among the parts of flowering plants were traced out in 
greater detail, but not in so radical a way; for Goethe did 
not, as did Wolff, verify his hypothesis by dissecting buds in 
their early stages of development. Goethe appears to have 
arrived at his conclusions independently. But that they were 
original with him, and that he gave a more variously-illus- 
trated exposition of them than had been given by Wolff, 
does not entitle him to anything beyond a secondary place, 
among those who have established this important generaliza- 
tion. 

Were it not that these pages may be read by some to 
whom Biology, in all its divisions, is a new subject of study, it 
would be needless to name the evidence on which this now- 
familiar generalization rests. For the information of such 
it will suffice to say, that the fundamental kinship existing 
among all the foliar organs of a flowering plant, is shown by 
the transitional forms which may be traced between them, 
and by the occasional assumption of one another’s forms. 
“Floral leaves, or bracts, are frequently only to be distin- 
guished from ordinary leaves by their position at the base of 
the flower; at other times the bracts gradually assume more 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. ra 9) 


and more of the appearance of the sepals.” The sepals, or 
divisions of the calyx, are not unlike undeveloped leaves: 
sometimes assuming quite the structures of leaves. In other 
cases, they acquire partially or wholly the colours of the 
petals—as, indeed, the bracts and uppermost stem-leaves 
occasionally do. Similarly, the petals show their alliances to 
the foliar organs lower down on the axis, and to those higher 
up on the axis: on the one hand, they may develop into or- 
dinary leaves that are green and veined; and, on the other 
hand, as so commonly seen in double flowers, they may bear 
anthers on their edges. All varieties of gradation into 
neighbouring foliar organs, may be witnessed in stamens. 
Flattened and tinted in various degrees, they pass insensibly 
into petals, and through them prove their homology with 
leaves; into which, indeed, they are transformed in flowers 
that become wholly foliaceous. The style, too, is occasionally 
changed into petals or into green leaflets; and even the 
ovules are now and then seen to take on leaf-lke forms. 
Thus we have clear evidence that in Phzenogams, all the ap- 
pendages of the axis are homplogues: they are all modified 
leaves. e 

Wolff established, and Goethe further illustrated, another 
general law of structure in flowering plants. ach leaf 
commonly contains in its axil, a bud, similar in structure to 
the terminal bud. This axillary bud may remain unde- 
veloped; or it may develop into a latera: shoot. like the 
main shoot; or it may develop into a flower. If a shoot 
bearing lateral flowers be examined, it will be found that the 
internode, or space which separates each leaf with its axillary 
flower from the leaf and axillary flower above it, becomes 
gradually less towards the upper end of the shoot. In some 
plants, as in the fox-glove, the internodes constitute a 
regularly-diminishing series. In other plants, the series they 
form suddenly begins to diminish so rapidly, as to bring the 
flowers into a short spike—instance the common orchis. And 


again, by a still more sudden dwarfing of the internodes, the 
3 * 


36 MORPHOLOGICAL DEVELOPMENT. 


flowers are brought into a cluster; as they are in the cows- 
lip. On contemplating a clover-flower, in which this 
clustering has been carried so far as to produce a com- 
pact head; and on considering what must happen if, by a 
further arrest of axial development, the foot-stalks of the 
florets disappear; it will be seen that there must result a 
crowd of flowers, seated close together on the end of the axis. 
And if, at the same time, the internodes of the upper stem- 
leaves also remain undeveloped, these: stem-leaves will be 
grouped into a common calyx or involucre: we shall have a 
composite flower, such as the thistle. Hence, to modifications 
in the developments of foliar organs, have to be added modi- 
fications in the developments of axial organs. Comparisons 
disclose the gradations through which axes, like their append- 
ages, pass into all varieties of size, proportion, and structure. 
And we learn that the occurrence of these two kinds of 
metamorphosis, in all conceivable degrees and combinations, 
furnishes us with a proximate interpretation of morpho- 
logical composition in Pheenogams. 

I say a proximate interpretation, because there remain 
to be solved certain deeper problems; one of which at once 
presents itself to be dealt with under the present head. 
Leaves, petals, stamens, &c., being shown to be homologous 
foliar organs; and the part to which they are attached, 
proving to be an indefinitely-extended axis of growth, or 
axial organ; we are met by the questions,— What is a foliar 
organ ? and What is an axial organ ? The morphological com- 
position of a Pheenogam is undetermined, so long as we can- 
not say to what lower structures leaves and shoots are homo- 
logous; and how this integration of them originates. To 
these questions let us now address ourselves. 


§ 190-1. Already, in § 78, reference has been made to the 
occasional development of foliar organs into axial organs: 
the special case there described, being that of a fox-glove, in 
which some of the sepals were replaced by flower-buds. 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 37—43 


The observation of these and some analogous monstrosities, 
raising the suspicion that: the. distinction between foliar 
organs and axial organs is-not absolute, led me to examine 
into the matter; and the result has been the deepening of 
this suspicion into a conviction. Part of the evidence is given 
in Appendix A. 

Some time after having reached this conviction, [ found on 
looking into the literature of the subject, that analogous ir- 
regularities have suggested to other observers, beliefs similarly 
at variance with the current morphological creed. Difli- 
culties in satisfactorily defining these two elements, have 
served to shake this creed in some minds. ‘To others, 
the strange leaf-like developments which axes undergo in 
certain plants, have afforded reasons for doubting the 
constancy of this distinction which vegetal morphologists 
usually draw. And those not otherwise rendered sceptical, 
have been made to hesitate by such cases as that of the 
Nepaul-barley ; in which the glume, a foliar organ, becomes 
developed into an axis, and bears flowers. In his essay— 
“Vegetable Morphology: its History and Present Condi- 
tion,” * whence I have already quoted, Dr Masters indicates 
sundry of the grounds for thinking, that there is no impassable 
demarcation between leaf and stem. Among other difficult- 
ies which meet us if we assume that the distinction is abso- 
lute, he asks—‘‘ What shall we say to cases such as those 
afforded by the leaves of Guarea and Trichilia, where the 
leaves after a time assume the condition of branches and de- 
velop young leaflets from their free extremities, a process less 
perfectly seen in some of the pinnate-leaved kinds of Berberis 
or Mahonia, to be found in almost every shrubbery ?” 

A class of facts on which it will be desirable for us here to 
dwell a moment, before proceeding to deal with the matter 
deductively, is presented by the Cactacee. In this remark- 
able group of plants, deviating in such varied ways from the 
ordinary phenogamic type, we find many highly instructive 


* See British and Foreign Medico-Chirurgical Review for January, 1862. 


44 MORPHOLOGICAL DEVELOPMENT. 


modifications of form and structure. By contemplating the 
changes here displayed within the limits of a single order, 
we shull greatly widen our conception of the possibilities of 
metamorphosis in the vegetal kingdom, taken as a whole. 
Two different, but similarly-significant, truths are illustrated. 
First, we are shown how, of these two components of a 
flowering plant, commonly regarded as primordially distin- 
guished, one may assume, throughout numerous species, the 
functions, and to a great degree the appearance, of the other, 
Second, we are shown how, in the same individual, there 
may occur a re-metamorphosis—the usurped function and 
appearance being maintained in one part of the plant, while 
in another part, there is a return to the ordinary appearance 
and function. We will consider these two truths separ- 
ately. Some of the Huphorbiacee, which simulate 
Cactuses, show us the stages through which such abnormal 
structures are arrived at. In Huphorbia splendens, the lateral 
axes are considerably swollen at their distal ends, so as often 
to be club-shaped: still, however, being covered with bark 
of the ordinary colour, and still bearing leaves. But 
in kindred plants, as Huphorbia nerivifolia, this swelling of 
the lateral axes is carried to a far greater extent; and, at 
the same time, a green colour and a fleshy consistence have 
been acquired: the typical relations nevertheless being still 
shown, by the few leaves that grow out of these soft and 
swollen axes. In the Cactacee, which are thus resembled by 
plants not otherwise allied to them, we have indications of a 
parallel transformation. Some kinds, not commonly brought 
to England, bear leaves; but in the species most familiar to 
us, the leaves are undeveloped and the axes assume their 
functions. Passing over the many varieties of form and 
combination which these green succulent growths display, we 
have to note that in some genera, as in PhyJlocactus, they 
become flattened out into foliaceous shapes, having mid-ribs 
and something approaching to veins. So that here, and in 
the genus Fniphyllwm, which has this character still more 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 45 


marked, the plant appears to be composed of fleshy 
leaves growing one upon another. And then, in Rhipsalis, 
the same parts are so leaf-like that an uncritical observer 
would regard them as leaves. These which are axial organs 
in their homologies, have become foliar organs in their 
analogies. When, instead of comparing these 
strangely-modified axes in different genera of Cactuses, we 
compare them in the same individual, we meet with transform- 
ations no less striking. Where a tree-like form is pro- 
duced by the growth of these foliaceous shoots, one on another ; 
and where, as a consequence, the first-formed of them become 
the main stem that acts as support to secondary and tertiary 
stems; they lose their green, succulent character, acquire 
bark, and become woody—in resuming the functions of axes 
they resume the structures of axes, from which they had de- 
viated. In Fig. 71 are shown some of the leaf-like axes of 
Ehipsalis rhombea in their young state; while Fig. 72 repre- 
sents the oldest portion of the 
same plant, in which the foli- 
aceous characters are quite 
obliterated, and there has re- 
sulted an ordinary stem-struc- 
ture. One further Fe 
fact is to be noted. At the 
same time that their leaf-like appearances are lost, the 
axes also lose their separate individualities. As they become 
stem-like, they also become integrated; and they do this so 
effectually, that their original points of junction, at first so 
strongly marked, are effaced, and a consolidated trunk is 
produced. 

Joined with the facts previously specified, these facts 
help us to conceive how, in the evolution of flowering plants 
in general, the morphological components that were once 
distinct, may become extremely disguised. We may ration- 
ally expect that during so long a course of modification, 
much greater changes of form, and much more decided fusions 


eee MORPHOLOGICAL DEVELOPMENT. 


of parts, have taken place. Seeing how, in an individual 
plant, the single leaves pass into compound leaves, by the devel- 
opment of their veins into mid-ribs while their mid-ribs begin 
to simulate axes; and seeing that leaves ordinarily exhibit- 
ing definitely-limited developments, occasionally produce 
other leaves from their edges; we are led to suspect the pos- 
sibility of still greater changes in foliar organs. When, fur- 
ther, we find that within the limits of one natural order, 
petioles usurp the functions and appearances of leaves, at the 
same time that in other orders, as in Ruscus, lateral axes so 
completely simulate leaves that their axial nature would never 
have been supposed, did they not bear flowers on their mid- 
ribs or edges; and when, among Cactuses, we perceive that 
such metamorphoses and re-metamorphoses take place with 
great facility ; our suspicion that the morphological elements 
of Phenogams admit of profound transformations, is 
deepened. And then, on discovering how frequent are the 
monstrosities that do not seem satisfactorily explicable without 
admitting the development of foliar organs into axial organs ; 
we become ready to entertain the hypothesis, that during the 
evolution of the phenogamic type, the distinction between 
leaves and axes has arisen by degrees. , 

With our pre-conceptions loosened by such facts, and 
carrying with us the general idea which such facts suggest, 
let us now consider in what way the typical structure of a 
flowering plant may be interpreted. 


§ 192. To proceed methodically, we must seek a clue to 
the structures of Endogens and Exogens, in the structures 
of those inferior plants that approach to them—Acrogens. 
The various divisions of this class present, along with sundry 
characters which ally them with Thallogens, other charac- 
ters by which the phznogamic structure is shadowed forth. 
While some of the inferior Hepatice or Liverworts, severally 
consist of little more than a thallus-like frond; among the 
higher members of this group, and still more among the 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. A7 


Mosses and Ferns, we find a distinctly marked stem.* Some 
Acrogens have foliar expansions that are indefinite in their 
forms; and some have quite definitely-shaped leaves. Roots 
are possessed by all the more developed genera of the class ; 
but there are other genera, as Sphagnum, which have no 
roots. Here the fronds are thallus-like, in being formed of only 
a single layer of cells; and there a double layer gives them 
a more leaf-like character—a difference exhibited between 
closely-allied genera of one order, the Mosses. Equally varied 
are the developments of the foliar-organs in their detailed 
structures : now being without mid-ribs or veins ; now having 
mid-ribs but no veins ; now having both mid-ribs and veins. 
Where stem and leaves exist, their imperfect differentiation 
is shown by the fact, that in many cases the stem is covered 
by an epidermis containing stomata. Nor must we omit the 
similarly-significant circumstance, that whereas in the lower 
Acrogens, the reproductive elements are immersed here and 
there in the thallus-like frond ; they are, in the higher orders, 
seated in well-specialized and quite distinct fructifying 
organs, having analogies with the flowers of Phznogams. 
Thus, many facts imply that if the phenogamic type is to be 
analyzed at all, we must look among the Acrogens for its mor- 
phological components, and the manner of their integration. 

Already we have seen among the lower Cryptogamia, how 


* Schleiden, who chooses to regard as an axis, that which Mr Berkeley, with 
more obvious truth, calls a mid-rib, says :—‘ The flat stem of the Liverworts pre- 
sents many varieties, consisting frequently of one simple layer of thin-walled 
cells, or it exhibits in its axis the elements of the ordinary stem.” This passage 
exemplifies the wholly gratuitous hypotheses which men will sometimes espouse, 
to escape hypotheses they dislike. Schleiden, with the positiveness characteristic 
of him, asserts the primordial distinction between axial organs and foliar organs. 
In the higher Acrogens, he sees an undeniable stem. In the lower Acrogens, clearly 
allied to them by their fructification, there is no structure having the remotest 
resemblance to a stem. But to save his hypothesis, Schleiden calls that ‘‘a flat 
stem,”’ which is very obviously a structure in which stem and leaf are not differ- 
entiated. He is the more to be blamed for this unphilosophical assumption, since 


he is merciless in his strictures on the unphilosophical assumptions of other 
botanists. 


48 MORPHOLOGICAL DEVELOPMENT. 


as they become integrated and definitely limited, aggregates 
acquire the habit of budding out other aggregates, on reach- 
ing certain stages of growth. Cells produce other cells 
endogenously or exogenously; and fronds give origin to 
other fronds from their edges or surfaces. We have seen, too, 
that the new aggregates so produced, whether of the first 
order or the second order, may either separate or remain 
connected. Fissiparously-multiplying cells in some cases 
fly asunder, while in other cases they unite into threads or 
laminee or masses; and fronds originating proliferously from 
other fronds, sometimes when mature disconnect themselves 
from their parents, and sometimes continue attached to them. 
Whether they do or do not part, is clearly determined by 
their nutrition. If the conditions are such that they can 
severally thrive better by separating after a certain develop- 
ment is reached, it will become their habit then to separate ; 
since natural selection will favour the propagation of those 
which separate most nearly at that time. If, conversely, it 
profits the species for the cells or fronds to continue longer 
attached, which it can only do if their growth and subse- 
quent powers of multiplication are thereby increased ; it must 
happen, through the continual survival of the fittest, that 
longer attachment will become an established characteristic ; 
and by persistence in this process, permanent attachment 
will result, when permanent attachment is advantageous. 
That disunion is really a consequence of relative innu- 
trition, and union a consequence of relative nutrition, 
is clear, a posteriori. On the one hand, the separation 
of the new individuals, whether in germs or as developed 
ageregates, is a decaying away of the connecting tissue; 
and this implies that the connecting tissue has ceased 
to perform its function as a channe! of nutriment. On 
the other hand, where, as we see among Pheenogams, there 
is about to take place a separation of new individuals in 
the shape of germs, at the point. where the nutrition is the 
lowest, a sudden increase of nutrition will cause the impend- 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 49 


ing separation to be arrested ; and the fructifying elements 
will revert towards the ordinary form, and develop in con- 
nexion with the parent. Turning to the Acrogens, we 
find among them many indications of this transition from dis- 
continuous development to continuous development. Thus the 
Liverworts give origin to new plants by cells which they 
thféw off from their surfaces; as, indeed, we have seen that 
much higher plants do. “According to Bischoff,” says 
Schleiden, “both the cells of the stem (Jungermannia biden- 
tata) and those of the leaves (J. ewsecta) separate themselves 
as propagative cells from the plant, and isolated cells shoot 
out and develop while still connected with the parent plant 
into small cellular bodies (J. violacea), which separate from 
the plant, and grow into new plants, asin Minium androgynum 
among the Mosses.’’ Now in the way above explained, these 
propagative cells and proliferous buds, may continue de- 
veloping in connexion with the parent, to various degrees 
before separating ; or the buds which are about to become 
fructifying organs, may similarly, under increased nutrition, 
develop into young fronds. As Sir W. Hooker says of the 
male fructification in Jungermannia furcata,—“ It has the ap- 
pearance of being a young shoot or innovation (for in colour 
and texture I can perceive no ‘difference) rolled up into a 
spherical figure.” On finding in this same plant, that some- 
times the proliferously-produced frond buds out from itself 
another frond before separating from the parent, as shown in 
Fig. 46.; it becomes clear that this long-continued connexion, 
may readily pass into permanent connexion. And when 
we see how, even among Pheenogams, buds may either detach 
themselves as bulbils, or remain attached and become shoots ; 
we can scarcely doubt that among inferior plants, less de- 
finite in their modes of organization, such transitions must 
continually occur. 

Let us suppose, then, that Fig. 73 is the frond of some 
primitive Acrogen, similar in general characters to Junger- 


mannia epiphylla, Fig.43; bearing, like it, the fructifying buds 


VOL. Il. 4 


50 MORPHOLOGICAL DEVELOPMENT. 


on its upper surface, and having a slightly- 
marked mid-rib and rootlets. And sup- 
pose that, as shown, a secondary frond is 
proliferously produced from the mid-rib, 
and continues attached to it. Hvidently, 
the ordinary discontinuous development, 
can thus become a continuous developméht, 
only on condition that there is an adequate 
supply, to the secondary frond, of such 
materials as are furnished by the rootlets: 
the remaining materials being obtainable 
by itself from the air. Hence, that portion 
of the mid-rib lying between the secondary 
frond and the chief rootlets, having its 
function increased, will increase in bulk. 
An additional consequence will be, a 
greater concentration of the rootlets— 
there will be extra growth of those which 
are most serviceably placed. Observe, next, 
that the structure so arising, is likely to be 
maintained. Such a variation implying, 
as it does, circumstances especially favour- 
able to the growth of the plant, will give 
to the plant extra chances of leaving de- 
scendants; since the area of frond sup- 
ported by a given area of the soil, being 
greater than in other individuals, there 
may be a greater production of spores. And then, among 
the more numerous descendants thus secured by it, the varia-. 
tion will give advantages to those in which it recurs. Such 
a mode of growth having, in this manner, become established, 
let us ask what is next likely to result. If it becomes the 
habit of the primary frond to bear a secondary frond from its 
mid-rib, this secondary frond, composed of physiological 
units of the same kind, will inherit the habit ; and supposing 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. ib 


that the supply of mineral matters obtained by the rootlets 
suffices for the full development of the secondary frond, there 
is a likelihood that the growth from it of a tertiary frond, will’ 
become an habitual characteristic of the variety. Along with 
the establishment of such a tertiary frond, as shown in Fig. 
74, there must arise a further development of mid-rib in the 
primary frond, as well as in the secondary frond—a develop- 
ment which must bring with it a greater integration of the 
two; while, simultaneously, extra growth will take place in 
such of the rootlets as are most directly connected with this 
main channel of circulation. Without further explanation it 
will be seen, on inspecting Figs. 75 and 76, that there may 
in this manner result an integrated series of fronds, placed 
alternately on opposite sides of a connecting vascular struc- 
ture. That this connecting vascular structure will, as shown 
in the figures, become more distinct from the foliar surfaces as 
these multiply, is no unwarranted assumption; for we have 
seen in compound-leaved plants, how, under analogous con- 
ditions, mid-ribs become developed into separate supporting 
parts, which acquire some of the characters of axes while as- 
suming their functions. And now mark how clearly 
the structure thus built up by integration of proliferously- 
growing fronds, corresponds with the structure of the more- 
developed Jungermanniacee. Kach of the fronds successively 
produced, repeating the characters of its parent, will bear 
roots; and will bear them in homologous places, as shown. 
Further, the united mid-ribs having but very little rigidity, 
will be unable to maintain an erect position. Hence there 
will result the recumbent, continuously-rooted stem, which 
these types exhibit. Nay, the parallelism is more complete 
than the figures show. To avoid confusion, the fronds thus 
supposed to be progressively integrated, have been repre- 
sented as simple. But, as shown in Fig. 45, these lower 
types ordinarily have fronds which divide dichotomously, in 
such way that one division is larger than the other ; and this 


4 * 
LIBRARY 


UNIVERSITY OF ILLINOIS 


59 MORPHOLOGICAL DEVELOPMENT. 


is just the character of the successive leaves in the higher 
types. As shown in Fig. 47, each leaf is usually composed 
of two unequal lobes. 

A natural concomitant of the mode of growth here de- 
scribed, is, that the stem, while it increases longitudinally, 
increases scarcely at all transversely: hence the name 
Acrogens. Clearly the transverse development of a stem, is 
the correlative, partly of its function as a channel of circula- 
tion, and partly of its function as a mechanical support. 
That an axis may lift its attached leaves into the air, implies 
thickness and solidity proportionate to the mass of such 
leaves; and an increase of its sap-vessels, also proportionate 
to the mass of such leaves, is necessitated when the roots 
are all at one end and the leaves at the other. But in the 
generality of Acrogens, these conditions, under which arises 
the necessity for transverse growth of the axis, are absent, 
wholly or in great part. The stem habitually creeps below 
the surface, or lies prone upon the surface; and where it 
grows in a vertical or inclined direction, does this by at- 
taching itself to a vertical or inclined object. Moreover, 
throwing ovt rootlets, as it mostly does, at intervals through- 
out its length, it is not called upon in any considerable de- 
gree, to transfer nutritive materials from one of its ends to 
the other. Hence this peculiarity which gives their name 
to the Acrogens, is a natural accompaniment of the low 
degree of specialization reached in them. And that it is an 
incidental and not a necessary peculiarity, is demonstrated 
by two converse facts. On the one hand, in those higher 
Acrogens which, like the tree-ferns, lift large masses of 
foliage into the air, there is just as decided a transverse ex- 
pansion of the axis as in Exogens. On the other hand, in 
those Hxogens which, like the common Dodder, gain sup- 
port and nutriment from the surfaces over which they creep, 
there is no more lateral expansion of the axis than is habit- 
ual among Acrogens. Concluding, as we are thus fully justi- 
fied in doing, that the lateral expansion accompanying longi- 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 53 


tudinal extension, which is a general characteristic of 
Endogens and Exogens as distinguished from Acrogens, is 
nothing more than a concomitant of their usually-vertical 
growth ;* let us now go on to consider how vertical growth 
originates, and what are the structural changes it involves. 


§ 193. Plants depend for their prosperity mainly on air 
and light: they dwindle where they are smothered, and 
thrive where they can expand their leaves into free space 
and sunshine. Those kinds which assume prone positions, 
consequently labour under disadvantages in being habitually 
interfered with by one another—they are mutually shaded 
and mutually injured. Such of them, however, as happen, 
by variations in mode of growth, to get at all above the rest, 
are more likely to flourish and leave offspring than the rest. 
That is to say, natural selection will favour the more upright- 
growing forms: individuals with structures that lift them 
above the rest, are the fittest for the conditions; and by the 
continual survival of the fittest, such structures must become 
established. There are two essentially-different ways in 
which the integrated series of fronds above described, may 
be modified so as to acquire the stiffmess needful for main- 
taining perpendicularity. We will consider them separately. 

A thin layer of substance gains greatly in power of re- 
sisting a transverse strain, if it is bent round so as to form a 
tube—witness the difference between the pliability of a sheet 
of paper when outspread, and the rigidity of the same sheet 
of paper when rolled up. Engineers constantly recognize 


* I am indebted to Dr Hooker for pointing out further facts supporting this 
view. In his Flora Antarctica, he describes the genus Lessonia (see Fig. 37) and 
especially Z. ovata, as having a mode of growth simulating that of the Exogens. 
The tall vertical stem thickens as it grows, by the periodical addition of layers 
to its periphery. Among lichens, too, it seems that there is an analogous case. 
That even Thallogens should thus, under certain conditions, present a transversely- 
increasing axis, shows that there is nothing absolute in the character which gives 
the names to the two highest classes of plants, in contradistinction to the class 
nearest to them. 


54 MORPHOLOGICAI, DEVELOPMENT. 


this truth, in devising appliances by which the greatest 
strength shall be obtained at the smallest cost of material ; 
and among organisms, we see that natural selection habit- 
ually establishes structures conforming to the same principle, 
wherever lightness and stiffness are to be combined. The 
cylindrical bones of mammals and birds, and the hollow 
shafts of feathers, are examples. The lower plants, too, 
furnish cases where the strength needful for maintaining an 
upright position, is acquired by this rolling up of a flat 
| thallus or frond. In Fig. 77, 
we have an Alga which ap- 
proaches towards a tubular 
> distribution of substance ; and 
which has a consequent rigid- 
ity. Sundry common forms 
of lichen, having the thallus 
folded into a branched tube, 
still more decidedly display- 
ing the connexion between 
this structural arrangement 
and this mechanical advantage. And from the particular 
class of plants we are here dealing with—the Acrogens—a 
type is shown in Fig. 78, Riella helicophylla, similarly cha- 
racterized by a thin frond that is made stiff enough to stand, 
by an incurving which, though it does not produce a hollow 
cylinder, produces a kindred form. If, then, as we have 
seen, natural selection or survival of the fittest, will favour 
such among these recumbent Acrogens, as are enabled, by 
variations of their structures, to maintain raised postures ; 
it will favour the formation of fronds that curve round upon 
themselves, and curve round upon the fronds growing out 
of them. What, now, will be the result should such a 
modification take place in the group of proliferous fronds 
represented in Fig. 76? Clearly, the result will be a 
structure like that shown in Fig. 79. And if this inrolling 
becomes more complete, a form like Jungermannia cordifolia, 


IB MORPHOLOGICAL COMPOSITION OF PLANTS. 55 


represented in Fig. 80, will 
be produced. 

When the successive fronds 
are thus folded round so com- 
pletely that their opposite 
edges meet, these opposite 
edges will be apt to unite : not 
that they will grow together 
after being formed, but that 
they willdevelopin connexion; 80 
or, in botanical language, will become “adnate.” That foliar 
surfaces which, in their embryonic state, are in close contact, 
often join into one, is a familiar fact. It is habitually so 
with sepals or divisions of the calyx. In all campanulate 
flowers it is so with petals. And in some tribes of plants 
it is so with stamens. We are therefore well-warranted in 
inferring, that under the conditions above described, the suc- 
cessive fronds or leaflets will, by union of their remote edges, 
first at their points of origin, and afterwards higher up, 
form sheaths inserted one within another, and including the 
AXIS. This incurving of the successive fronds, 
ending in the formation of sheaths, may be accompanied by 
different sets of modifications. Supposing Fig. 81 to be a 
transverse section of such a type (a being the mid-rib, and 
b the expansion of an older frond; while c is a younger frond 
proliferously developed within it), there may begin two di- 
vergent kinds of changes, leading to two contrasted struc- 
tures. If, while frond continues to grow out of frond, the 
series of united mid-ribs continues to be the channel of circu- 
lation between the uppermost fronds and the roots—if, as a 
consequence, the compound mid-rib, or rudimentary axis, con- 
tinues to increase in size laterally; there will arise the series 
of transitional forms represented by the transverse sections 
82, 83, 84, 85; ending in the production of a solid axis, 
everywhere wrapped round by the foliar surface of the 
frond, as an outer layer or sheath. But if, on the other 


56 MORPHOLOGICAL DEVELOPMENT. 


hand, circumstances favour a form of plant which maintains 
its uprightness at the smallest cost of substance—if the 


vascular bundles.of each succeeding mid-rib, instead of re- 
maining concentrated, become distributed all round the tube 
formed by the infolded frond; then the structure eventually 
reached, through the transitional forms 86, 87, 88, 89, will 
be a hollow cylinder. And now observe how the 
two structures thus produced, correspond with two kinds 
of Endogens. Fig. 90 represents a species of Dendrobium, 
in which we see clearly how each leaf is but a continuation 
of the external layer of a solid axis—a sheath such as would 
result from the infolded edges of a frond becoming adnate ; 
and on examining how the sheath of each leaf includes the 
one above it, and how the successive sheaths include the axis, 
it will be manifest that the relations of parts are just such 
as exist in the united series of fronds shown in Fig. 79—the 
successive nodes answering to the successive points of origin 
of the fronds. Conversely, the stem of a grass, Fig. 91, dis- 
plays just such relations of parts, as would result from the de- 
velopment of the type shown in Fig. 79, if instead of the mid- 
ribs thickening into a solid axis, the matter composing them 
became evenly distributed round the foliar surfaces, at the 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. OF 


same time that the incurved edges of the foliar surfaces 
united. The arrangements of the tubular axis and its ap- 
pendages, thus resulting, are still more instructive than those 
of the solid axis. For while, even more clearly than in the 
Dendrobium, we see at the point b, a continuity of structure 
between the substance of the axis below the node, and the 
substance of the sheath above the node; we see that this 
sheath, instead of having its edges united as in Dendrobium, 
has them simply overlapping, so as to form an incomplete 
hollow cylinder which may be taken off and unrolled; 


and we see that were the overlapping edges of this sheath 
united all the way from the node a to the node 3, it would 
constitute a tubular axis, like that which precedes it or like 
that which it includes. And then, giving an unexpected 
conclusiveness to the argument, it turns out that in one 
family of grasses, the overlapping edges of the sheaths do 
unite: thus furnishing us with a demonstration that tubular 
structures are produced by the incurving and joining ot 
foliar surfaces; and that so, hollow axes may be interpreted 
as above, without making any assumption unwarranted by 
fact. One further correspondence between the 
type thus ideally constructed, and the endogenous type, must 
be noted. If, as already pointed out, the transverse growth of 


38 MORPHOLOGICAL DEVELOPMENT. 


an axis arises, when the axis comes to be a channel of circu- 
lation between all the roots at one of its extremities and all 
the leaves at the other; and if this lateral bulging must in- 
crease, as fast as the quantity of foliage to be brought in 
communication with the roots increases—especially if such 
foliage has at the same time to be raised high above the 
earth’s surface ; what must happen to a plant constructed in 
the manner just described? The elder fronds or foliar or- 
gans, ensheathing those within them, as well as the incipient 
axis serving as a bond of union, are at first of such circum- 
ference only as suffices to inclose these undeveloped parts. 
What, then, will take place when the inclosed parts grow— 
when the axis thickens while it elongates? Evidently the 
earliest-formed sheaths, not being large enough for the 
swelling axis, must burst; and evidently each of the later- 
formed sheaths must, in its turn, do the like. There must 
result a gradual exfoliation of the successive sheaths, like 
that indicated as beginning in the above figure of Dendro- 
bium ; which, at a, shows the bud of the undeveloped parts 
just visible above the enwrapping sheaths, while at b, and c, 
it shows the older sheaths in process of being split open. 
That is to say, there must result the mode of growth which 
helps to give the name Endogens to this class. 

The other way in which an integrated series of fronds 
may acquire the rigidity needful for maintaining an erect 
position, has next to be considered. If the successive fronds 
do not acquire such habit of curling as may be taken ad- 
vantage of by natural selection, so as to produce the requisite 
stiffness ; then, the only way in which the requisite stiffness 
appears producible, is by the thickening and hardening of 
the fused series of mid-ribs. The incipient axis will not, in 
this case, be inclosed by the rolled-up fronds; but will con- 
tinue exposed. Survival of the fittest will favour the genesis 
of a type, in which those portions of the successive mid-ribs 
that enter into the continuous bond, become more bulky than 
the disengaged portions of the mid-ribs: the individuals 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 59 


which thrive and have the best chances of leaving offspring, 
veing, by the hypothesis, individuals having axes stiff 
enough to raise their foliage above that of their fellows. 
At the same time, under the same influences, there will tend 
to result an elongation of those portions of the mid-ribs, 
which become parts of the incipient axis ; seeing that it will 
profit the plant to have its leaves so far removed from one 
another, as to prevent mutual interferences. Hence, from the 
recumbent type, there will evolve, by indirect equilibration, 
(§ 167) such modifications as are shown in Figs. 92, 93, 94: 


92 


\ 


the first of which is a slight advance on the ideal type 
represented in Fig. 76, arising in the way described; and 
the others of which are actual plants—Jungermannia Hookert, 
and J. decipiens. Thus the higher Acrogens show us how, 
along with an assumption of the upright attitude, there does 
g0 on, as we see there must go on, a separation of the leaf- 
producing parts from the root-producing parts; a greater 
development of that connecting portion of the successive 
fronds, by which they are kept in communication with the 
roots, and raised above the ground; and a consequent in- 
creased differentiation of such connecting portion from the 
parts attached to it. And this lateral bulging of the axis, 
directly or indirectly consequent on its functions as a support 


60 MORPHOLOGICAL DEVELOPMENT. 


and a channel, being here unrestrained by the early-formed 
fronds folded round it, goes on without the bursting of these. 
Hence arises a leading character of what is called exogenous 
erowth—a growth which is, however, still habitually accom- 
panied by exfoliation, in flakes, of the outermost layer, con- 
tinually being cracked and split by the accumulation of 
layers within it. And now if we examine plants 
of the exogenous type, we find among them many displaying 
the stages of this metamorphosis. In Fig. 95, is shown a 
form in which the continuity of the axis with the mid-rib of 
the leaf, is manifest—a continuity that is conspicuous in the 
common thistle. Here the foliar expansion, running some 
distance down the axis, makes the included portion of the 


=) ER 
ANS 


axis a part of its mid-rib; just as in the ideal types above 
drawn. By the greater growth of the internodes, which are 
very variable, not only in different plants but in the same 
plant, there results a modification like that delineated in 
Fig. 96. And then, in such forms as Fig. 97, there is shown 
the arrangement that arises when, by more rapid develop- 
ment of the proximal portion of the mid-rib, the distal part 
of the foliar surface is separated from the part which em- 
braces the axis: the wings of the mid-rib still serving, how- 
ever, to connect the two portions of the foliar surface. Such 
a separation is, as pointed out in § 188, an habitual occur- 
rence ; and in some compound leaves, an actual tearing of the 
inter-veinous tissue, is caused by extra growth of the mid-rib. 
Modifications like this, and the further one in Fig. 98, we 
may expect to be established by survival of the fittest, among 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 61 


those plants which produce considerable masses of leaves; 
since the development of mid-ribs into footstalks, by throw- 
ing the leaves further away from the axes, will diminish the 
shading of the leaves, one by another. And then, among 
plants of bushy growth, in which the assimilating surfaces 
become still more liable to intercept one another’s light, 
natural selection will continue to give an advantage to those 
which carry their assimilating surfaces at the ends of the 
petioles, and do not develop assimilating surfaces close to 
the axis, where they are most shaded. Whence will result 
a disappearance of the stipules and the foliar fringes of the 
-mid-ribs ; ending in the production of the ordinary stalked 
leaf, Fig. 99, which is characteristic of trees. Meanwhile, 
the axis thickens in proportion to the number of leaves it 
has to carry, and to put in communication with the roots ; 
and so there comes to be a more marked contrast between it 
and the petioles, severally carrying a leaf each.* 


§ 194. When, in the course of the process above sketched 
out, there has arisen such community of nutrition among the 
fronds thus integrated into a series, that the younger ones 
are aided by materials which the older ones have elaborated ; 
the younger fronds will begin to show, at earlier and earlier 
periods of development, the structures about to originate 
from them. Abundant nutrition will abbreviate the intervals 
between the successive prolifications; so that eventually, 
while each frond is yet imperfectly formed, the rudiment of 
the next will begin to show itself. All embryology justifies 
this inference. The analogies it furnishes lead us to expect 
that when this serial arrangement becomes organic, the 
growing part of the series will show the general relations of 


* Since this paragraph was put in type, I have observed that in some varieties 
of Cineraria, as probably in other plants, a single individual furnishes all these 
forms of leayes—all gradations between unstipulated leaves on long petioles, and 
leaves that embrace the-axis. It may be added that the distribution of these va- 
rious forms, is quite in harmony with the rationale above given. 


62 MORPHOLOGICAL DEVELOPMENT. 


the forthcoming parts, while they are very small and un- 
specialized. What will in such case be the appearances they 
assume? We shall have no difficulty in perceiving what it 
will be, if we take a form like that shown im Fig. 92, and 
dwarf its several parts at the same time that we generalize 
them. Figs. 100, 101, 102, and 108, will show the result ; 
and in Fig. 104, which is the bud of an exogen, we see how 


Gf SONATAS, 


clear is the morphological correspondence: a being the 
rudiment of a foliar organ beginning to take shape ; b being 
the almost formless rudiment of the next foliar organ; and 
c being the quite-undifterentiated part whence the rudiments 
of subsequent foliar organs are to arise. 

And now we are prepared for entering on a still-remaining 
question respecting the structure of Phanogams—what is the 
origin of axillary buds? As the synthesis at present stands, 
it does not account for these; but on looking a little more 
closely into the matter, we shall find that the axillary buds 
are interpretable in the same manner as the terminal buds. 
So to interpret them, however, we must return to that pro- 
cess of proliferous growth with which we set out, for the pur- 
pose of observing some facts not before named. Delesseria 
hypoglossum, Fig. 105, represents a seaweed of the same genus 
as one outlined in Fig. 40; but of a species in which pro- 
liferous growth is carried much further. Here, not only does 
the primary frond bud out many secondary fronds from its 
mid-rib ; but most of the secondary fronds similarly bud out 
several tertiary fronds; and even by some of the tertiary 
fronds, this prolification is repeated. Besides being shown 
that the budding out of several fronds from one frond, may 
become habitual; we are also shown that it may become a 


habit inherited by the fronds so produced, and also by the 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 63 


fronds they produce: the manifestation of the tendency, 
being probably limited only by failure of nutrition. That 
under fit conditions, an analogous mode of growth will occur 
in fronds of the acrogenic type, like those we set out with, is 
shown by the case of Jungermannia furcata, Figs. 45, 46, in 
which such compound prolification is partially displayed. 
Let us suppose then, that the frond a, Fig. 106, produces 


not only a single secondary frond b, but also another such 
secondary frond, U'. Let us suppose, further, that the frond 
b is in like manner doubly proliferous: producing both 
and cl. Lastly, let us suppose that in the second frond }! 
which a@ produces, as well as in the second frond c! which b 
produces, the doubly-proliferous habit is manifested. If, 
now, this habit grows organic—if it becomes, as it natur- 
ally will become, the characteristic of a plant of luxuriant 
growth, the unfolding parts of which can be fed by the un- 
folded parts; it will happen with each lateral series, as with 
the main series, that its successive components will begin to 
show themselves at earlier and earlier stages of development. 
And in the same way that, by dwarfing and generalizing 


64 MORPHOLOGICAL DEVELOPMENT. 


the original series, we arrive at a structure like that of the 
terminal bud; by dwarfing and generalizing a lateral series, 
as shown in Figs. 107—110, we arrive at a structure an- 
swering in nature and position to the axillary bud. 


SS AL al al 


Facts confirming these interpretations, are afforded by 
the structure and distribution of buds. The phzenogamic 
axis in its primordial form, being an integrated series of 
folia ; and the development of that part by which these folia 
are held together at considerable distances from one another, 
taking place afterwards; it is inferable from the general 
principles of embryology, that in its rudimentary stages, the 
pheenogamic axis will have its foliar parts much more clearly 
marked out than its axial parts. This we see in every bud. 
Every bud consists of the rudiments of leaves packed to- 
gether without any appreciable internodal spaces; and the 
internodal spaces begin to increase with rapidity, only when 
the foliar organs have been considerably developed. More- 
over, where nutrition is defective, and arrest of development 
takes place—that is, where a flower is formed—the inter 
nodes remain undeveloped: the process of unfolding ceases 
before the later-acquired characters of the phenogamic axis 
are assumed. Lastly, as the hypothesis leads us to expect, 
axillary buds make their appearances later than the foliar 
organs which they accompany ; and where, as at the ends of 
axes, these foliar organs show failure of chlorophyll, the — 
axillary buds are not produced at all. That these are in- 
ferable traits of structure, will be manifest on contemplating 
Figs. 106—110; and on observing, first, that the doubly- 
proliferous tendency of which the axillary bud is a result, im- 
plies abundant nutrition; and on observing, next, that the 
original place of secondary prolification, is such that the foliar 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 65 


surface on which it occurs, must grow to some extent before 
the bud appears. 

On thus looking at the matter—on aeataeanlannie afresh 
the ideal type shown in Fig. 106, and noting how, by the 
conditions of the case, the secondary prolifications must cease 
before that primary prolification which produces the main 
axis; we are enabled to reconcile all the phenomena of axil- 
lary gemmation. We see harmony among the several facts— 
first, that the axillary bud becomes a lateral, leaf-bearing 
axis if there is abundant material for growth; second, that 
its development is arrested, or it becomes a flower-bearing 
axis, if the supply of sap is but moderate; third, that it is 
absent when the nutrition is failing. We are no longer 
committed to the gratuitous assumption, that in the pheeno- 
gamic type, there must exist an axillary bud to each foliar 
organ; but we are led to conclude, a priori, that which we 
find, a posteriori, that axillary buds are as normally absent 
in flowers as they are normally present lower down the 
axis. And then, to complete the argument, we are prepared 
for the corollary that axillary prolification may naturally 
arise even at the ends of axes, provided the failing nutrition 
which causes the dwarfing of the foliar organs to form a 
flower, be suddenly changed into such high nutrition as to 
transform the components of the flower into appendages 
that are green, if not otherwise leaf-like—a condition under 
which only, this phenomenon is proved to occur. 


§ 195. One more question presents itself, when we con- 
trast the early stages of development in the two classes of 
Phenogams; and a further answer supplied by the hypothe- 
sis, gives to the hypothesis a further probability. It is cha- 
racteristic of an endogen, to have a single seed-leaf or coty- 
ledon ; and it is characteristic of an exogen, to have at least 
two cotyledons, if not more than two. That is to say, the 
monocotyledonous mode of germination everywhere co- 


exists with the endogenous mode of growth; and along with 
VOL, II. a 


66 MORPHOLOGICAL DEVELOPMENT. 


the exogenous mode of growth, there always goes either a 
dicotyledonous or polycotyledonous germination. Why is 
this? Such correlations cannot be accidental—cannot be 
meaningless. A true theory of the phenogamic types, in 
their origin and divergence, should account for the cornex- 
ion of these traits. Let us see whether the foregoing theory 
does this. 

The higher plants, like the higher animals, bequeath to 
their offspring more or less of nutriment and structure. 
Superior organisms of either kingdom do not, as do all in- 
ferior organisms, cast off their progeny in the shape of 
minute portions of protoplasm, unorganized and without 
stocks of material fit for them to organize; but they either 
deposit along with the germs they cast off, certain quantities 
of albumenoid substance, fit for them to appropriate while 
they develop themselves, or else they continue to supply such 
substance while the germs partially-develop themselves before 
their detachment. Among plants, this constitutes the dis- 
tinction between seeds and spores. Every seed contains a 
store of food to serve the young plant during the first stages 
of its independent life; and usually, too, before the seed is 
detached, the young plant is so far advanced in structure, 
that it bears to the attached stock of nutriment much the 
same relation that the young fish bears to the appended yelk- 
bag at the time of leaving the egg. Sometimes, indeed, the 
development of chlorophyll gives the seed-leaves a bright 
green, while the seed is still contained in the parent- 
pod. This early organization of the pheeno- 
gam, must be supposed rudely to indicate the type out of 
which the phznogamic type arose. On the foregoing hypo- 
thesis, the seed-leaves therefore represent the primordial 
fronds—which, indeed, they simulate in their simple, cellular, 
unveined structures. And the question here to be asked is— 
do the different relations of the parts in young endogens and 
exogens correspond with the different relations of the primor- 
dial fronds, severally implied by the endogenous and the 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 67 


exogenous modes of growth? We shall find that they do. 

Starting, as before, with the proliferous form shown in 
Fig. 111, it is clear that if the strength required for main- 
taining the vertical attitude, is obtained by the rolling up of 
the fronds, the primary frond will more and more conceal the 
secondary frond within it. At the same time, the secondary 
frond must continue to be dependent on the first for its nutri- 
tion ; and being produced. within the first, must be prevented 
by defective supply of light and air, from ever becoming syn- 
chronous in its development with the first. Hence, this 
infolding which leads to the endogenous mode of growth, 
implies that there must always continue such pre-eminence 


fh) 
47) 


a: 
{ 126 
421 


of the first-formed frond or its representative, as to make the 

germination monocotyledonous. Figs. 111 to 115, show the 

transitional forms that would result from the infolding of 

the fronds. In Fig, 116, a vertical section of the form repre- 

sented in Fig. 115, are exhibited the relations of the succes- 
5 * 


68 MORPHOLOGICAL DEVELOPMENT. 


sive fronds to each other. The modified relations that would 
result, if the nutrition of the embryo admitted of anticipatory 
development of the successive fronds, are shown in Fig. 117. 
And how readily the structure may pass into that of the 
monocotyledonous germ, will be seen on inspecting Fig. 118; 
which is a vertical section of an actual monocotyledon at an 
early stage—the incomplete lines at the left of its root, indi- 
cating its connexion with the seed.* Contrariwise, 
where the strength required for maintaining an upright atti- 
tude is not obtained by the rolling up of the fronds, but by 
the strengthening of the continuous mid-rib, the second 
frond, so far from being less favourably circumstanced than 
the first, becomes in some respects even more favourably 
circumstanced: being above the other, it gets a greater share 
of light, and it is less restricted by surrounding obstacles. 
There is nothing, therefore, to prevent it from rapidly gaining 
an equality with the first. And if we assume, as the truths of 
embryology entitle us to do, an increasing tendency towards 
anticipation in the development of subsequent fronds—if 
we assume that here, as in other cases, structures which 
were originally produced in succession, will, if the nutrition 
allows and no mechanical dependence hinders, come to be pro- 
duced simultaneously ; there is nothing to prevent the pas- 
sage of the type represented in Fig. 111, into that represented 


* Since these figures were put on the block, it has occurred to me that the 
relations would be still clearer, were the primary frond represented as not taking 
part in these processes of modification, which have been described as giving rise 
to the erect form; as, indeed, the rooting of its under surface will prevent it from 
doing in any considerable degree. In such case, each of the Figs. 111 to 117, 
should have a horizontal rooted frond at its base, homologous with the pro-em- 
bryo among Acrogens. This primary frond would then more manifestly stand in 
the same relation to the rest, as the cotyledon does to the plumule—both by 
position, and as a supplier of nutriment. Fig. 117 a, which I am enabled to 
add, shows that this would complete the interpretation. Of the dicotyledonous 
series, it is needful to add no further explanation than that the difference in habit 
of growth, will permit the second frond to root itself as well as the first; and so 
to become an additional source of nutrition, similarly circumstanced to the first 
and equal with it. 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 69 


in Fig. 122. Or rather, there is everything to facilitate it ; 
seeing that natural selection will continually favour the pro- 
duction of a form in which the second frond grows in such 
way as not to shade the first, and in such way as allows the 
axis readily to assume a vertical position. | 

Thus, then, is interpretable the universal connexion between 
monocotyledonous germination and endogenous growth; as 
well as the similarly-universal connexion between exogenous 
growth and the development of two or more cotyledons. 
That it explains these fundamental relations, adds very 
greatly to the probability of the hypothesis. 


§ 196. While we are in this manner enabled to discern 
the kinship that exists between the higher vegetal types 
themselves, as well as between them and the lower types; we 
are at the same time supplied with a rationale of those truths 
which vegetal morphologists have established. Those homo- 
logies which Wolff indicated in their chief outlines and 
Goethe followed out in detail, have a new meaning given to 
them when we regard the phenogamic axis as having been 
evolved in the way described. Forming the modified con- 
ception which we are here led to do, respecting the units of 
which a flowering plant is composed, we are no longer left 
without an answer to the question— What is an axis? And 
we are helped to understand the naturalness of those cor- 
respondences which the successive members of each shoot 
display. Let us glance at the facts from our present stand- 
point. 

The unit of composition of a Phenogam, is such portion of 
a shoot as answers to one of the primordial fronds. This 
portion is neither one of the foliar appendages nor one of the 
internodes; but it consists of a foliar appendage together 
with the preceding internode, including the axillary bud 
where this is developed. The parts intercepted by the dotted 
lines in Fig. 128, constitute such a segment; and the true 
homology is between this and any other foliar organ with the 


70 MORPHOLOGICAL DEVELOPMENT. 


portion of the axis below it. And now observe how, when we 
take this for the unit of composition, the metamorphoses 
which the phzenogamic axis displays, are inferable from known 
laws of development. Embryology teaches us that arrest 
of development shows itself first in the absence of those parts 
that have arisen latest in the course of evolution; that if 
defect of nutrition causes an earlier arrest, parts that are of 
more ancient origin abort; and that the part alone produced 
when the supply of materials fails near the outset, is the prim- 
ordial part. We must infer, therefore, that in each seg- 
ment of a Phenogam, the foliar organ, which answers to the 
primordial frond, will be the most constant element; and 
that the internode and the axillary bud, will be successively 
less constant. This we find. Along with a smaller size of 
foliar surface implying lower nutrition, it is usual to see a 
much-diminished internode and a less-pronounced axillary 
bud, as in Fig. 124. On approaching the flower, the 


1) 2° te 


127 128 129 


axillary bud disappears; and the segment is reduced to 
a small foliar surface, with an internode which is in most 
cases very short if not absent, as in 125 and 126. In the 
flower itself, axillary buds and internodes are both want- 
ing: there remains only a foliar surface (127), which, | 
though often ‘larger than the immediately preceding foliar 
surface, shows failing nutrition by absence of chlorophyll. 
And then, in the quite terminal organs of fructification (129), 
we have the foliar part itself reduced to a mere rudiment. 
Though these progressive degenerations are by no means 
regular, being in many cases varied by adaptation to par- 
ticular requirements, yet it cannot, I think, be ‘questioned, 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 71 


that the general relations are as described, and that they are 
_ such as the hypothesis leads us to expect. Nor are 
we without a kindred explanation of certain remaining traits 
of foliar organs in their least-developed forms. Petals, 
stamens, pistils, &c., besides reminding us of the primordial 
fronds by their diminished sizes, and by the want of those 
several supplementary parts which the preceding segments 
possess, also remind us of them by their histological charac- 
ters: they consist of simple cellular tissue, scarcely at all 
differentiated. The fructifying cells, too, which here make 
their appearance, are borne in ways like those in which the 
lower Acrogens bear them—at the edge of the frond, or at 
the end of a peduncle, or immersed in the general substance ; 
as in Figs. 128 and 129. Nay, it might even be said that. 
the colours assumed by these terminal folia, call to mind the 
plants out of which we conclude that Phzenogams have been 
evolved ; for it is said of the fronds of the Jungermanniacee, 
that ‘‘though under certain circumstances of a pure green, 
they are inclined to be shaded with red, purple, chocolate, or 
other tints.” 

As thus understood, then, the homologies among the parts 
of the phenogamic axis are interpretable, not as due to a 
needless adhesion to some typical form or fulfilment of a pre- 
determined plan; but as the inevitable consequences of the 
mode in which the phzenogamic axis originates. 


§ 197. And now it remains only to observe, in confirmation 
of the foregoing synthesis, that it at once explains for us 
various irregularities. When we see leaves sometimes pro- 
ducing leaflets from their edges or extremities, we recognize 
in the anomaly, a resumption of an original mode of growth : 
fronds frequently do this. When we learn that a flowering 
plant, as the Drosera intermedia, has been known to develop 
a young plant from the surface of one of its leaves, we are at 
once reminded of the proliferous growths and fructifying 
organs in the Liverworts. The occasional production of bul- 


72 MORPHOLOGICAL DEVELOPMENT, 


bils by Phenogams, ceases to be so surprising when we find 
it to be habitual among the inferior Acrogens; and when we 
see that it is but a repetition, on a higher stage, of that self- 
detachment which is common among proliferously-produced 
fronds. Nor are we any longer without a solution of that 
transformation of foliar organs into axial organs, which 
not uncommonly takes place. How this last irregularity 
of development is to be accounted for, we will here pause a 
moment to consider. Let us first glance at our data, 

The form of every organism, we have seen, must depend 
on the structures of its physiological units. Any group of 
such physiological units will tend to arrange itself into the 
complete organism, if it is uncontrolled and placed in fit 
conditions. Hence the development of fertilized germs; and 
hence the development of those self-detached cells which 
characterize some plants. Conversely, physiological units 
which form a small group involved in a larger group, and are 
subject to all the forces of the larger group, will become sub- 
ordinate in their structural arrangements to the larger group 
—will be co-ordinated into a part of the major whole, in- 
stead of co-ordinating themselves into a minor whole. This 
antithesis will be clearly understood on remembering how, 
on the one hand, a small detached part of a hydra soon 
moulds itself into the shape of an entire hydra; and how, 
on the other hand, the cellular mass that buds out in place 
of a lobster’s lost claw, gradually assumes the form of a claw 
—has its parts so moulded as to complete the structure of 
the organism: a result which we cannot but aseribe to the 
forces which the rest of the organism exerts upon it. Con- 
sequently, among plants, we may expect that whether any 
portion of protoplasm moulds itself into the typical form 
around an axis of its own, or is moulded into a part subor- 
dinate to another axis, will depend on the relative mass of 
its physiological units—the accumulation of them that has 
taken place before the assumption of any structural arrange- 
ment. A few illustrations will make clear the validity of 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. to 


this inference. In the compound leaf, Fig. 65, the several 
lateral growths a, b, c, d, are manifestly homologous ; and 
on comparing a number of such leaves together, it’ will be 
seen that one of these lateral growths may assume any de- 
gree of complexity, according to the degree of its nutrition. 
Every fern leaf exemplifies the same general truth still bet- 
ter. Whether each sub-frond remains an undeveloped wing 
of the main frond, or whether it organizes itself into a group 
of frondlets borne by a secondary rib, or whether, going 
further, as it often does, it gives rise to tertiary ribs, is 
clearly determined by the supply of materials for growth ; 
since such higher developments are habitually most marked 
at points where the nutrition is greatest; namely, next the 
stem. But the clearest evidence is afforded among the Alga, 
which, not drawing nutriment from roots, have their parts 
much less mutually dependent; and are therefore capable of 
showing more clearly, how any part may remain an append- 
age or may become the parent of appendages, according to 


circumstances. In the annexed Fig. 180, He 
representing a branch of Ptilota pluwmosa, cle 


we see how a wing grows into a wing-bear- dt Ssviraty ¢4s 
ing branch, if its nutrition passes a certain oe oe 
point. This form, so strikingly like that of “Sycaurees 
the feathery erystallizations of many inor- 
ganic substances, proves to us that, as in 
such crystallizations, the simplicity or com- 
plexity of structure at any place, depends OE 
on the quantity of matter that has to be 
polarized at that place in a given time.* 


> 


* How the element of time modifies the result, is shown by the familiar fact that 
crystals rapidly formed are small; and that they become larger when they are 
formed more slowly. If the quantity of molecules contained in a solution is rela- 
tively great, so that the mutual polarities of the molecules crowded together in 
every place throughout the solution are intense, there arises a crystalline aggre- 
gation around local axes; whereas, in proportion as the local action of molecules 
on one another is rendered less intense by their wider dispersion, they become 


74 MORPHOLOGICAL DEVELOPMENT. 


Hence, then, we are not without an interpretation of those 
over-developments which the phenogamic axis occasionally 
undergoes. Fig. 104, represents the pheenogamic bud in its 
rudimentary state. The lateral process b, which ordinarily 
becomes a foliar appendage, differs very little from the 
terminal process c, which is to become an axis—differs 
mainly in having, at this period when its form is being 
determined, a smaller bulk. If while thus undifferentiated, 
its nutrition remains inferior to that of the terminal process, 
it becomes moulded into a part that is subordinate to the 
general axis. But if, as sometimes happens, there is supplied 
to it such an abundance of the materials needful for growth, 
that it becomes as large as the terminal process; then we 
may naturally expect it to begin moulding itself round an 
axis of its own: a foliar organ will be replaced by an axial 
organ. And this result will be especially lable to occur, 
when the growth of the axis has been previously under- 
going that arrest which leads to the formation of a flower; 
that is, when, from defect of materials, the terminal process 
has almost ceased to increase, and when some concurrence of 
favourable causes, brings a sudden access of sap, which reaches 
the lateral processes before it reaches the terminal process. 


§ 198. The general conclusion to which these various lines 
of evidence converge, is, then, that the shoot of a flowering 
plant is an aggregate of the third degree of composition. 
Taking as aggregates of the first order, those small masses 
of protoplasm which ordinarily assume the forms under 
which they are known as cells; and considering as aggregates 
of the second order, those assemblages of such cells which, 
in the lower cryptogamia, compose the various kinds of thal- 
lus; then that structure, common to the higher cryptogams 

and to phznogams, in which we find a series of such groups 


relatively more subordinate to the forces exerted on them by the larger aggre- 
gates of molecules that are at greater distances, and thus are left to arrange 
themselves round fewer axes into larger crystals, 


THE MORPHOLOGICAL COMPOSITION OF PLANTS. 75 


of cells bound up into a continuous whole, must be regarded 
as an aggregate of the third order. The inference drawn 
from analysis, and verified by a synthesis that corresponds in 
a remarkable manner with the facts, is, that those compound 
parts which, in Endogens and Iixogens, are called axes, 
have really arisen by integration of such simple parts as in 
lower plants are called fronds. Here, on a higher level, ap- 
pears to have taken place a repetition of the process already 
observed on lower levels. The formation of those small 
groups of physiological units which compose the lowest 
protophytes, is itself a process of integration ; and the con- 
solidation of such groups into definitely-circumscribed and 
coherent cells, is a completing of the process. In those 
coalescences, variously carried on, by which many such cells 
are joined into threads, and discs, and solid or flattened- 
out masses, we see these morphological units aggregating 
into units of a compound kind—the different phases of the 
transition being exemplified by groups of various sizes, 
various degrees of cohesion, and various degrees of definite- 
ness. Once more do we now find evidences of a like process 
on a larger scale: the compound groups are again com- 
pounded. And, as before, there are not wanting types of 
organization by which the stages of this higher integration 
are shadowed forth. From fronds that occasionally produce 
other fronds from their surfaces, we pass to those that 
habitually produce them. From those that do so in an in- 
definite manner, to those that do so in a definite manner. 
And from those that do so singly, to those that do so doubly 
and triply through successive generations of fronds. Even 
within the limits of a sub-class, we find gradations between 
fronds irregularly proliferous, and groups of such fronds 
united into a regular series. 

Nor does the process end here. The flowering plant is 
rarely uniaxial—it is nearly always multiaxial. From its 
primary shoot, there grow out secondary shoots of like kind. 
Though occasionally among Phaenogams, and frequently 


76 MORPHOLOGICAL DEVELOPMENT. 


among the higher Cryptogams, the germs of new axes detach 
themselves under the form of bulbils, and develop separately 
instead of in connexion with the parent axis; yet in most 
Phenogams, the germ of each new axis maintains its con- 
nexion with the parent axis: whence results a group of axes 
—an aggregate of the fourth order. Every tree, by the pro- 
duction of branch out of branch, shows us this integration 
repeated over and over again: forming an aggregate having 
a degree of composition too complex to.be any longer defined. 


CHAPTER IV. 
THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 


§ 199. Wuar was said in § 180, respecting the ultimate 
structure of organisms, holds more manifestly of animals 
than of plants. That throughout the vegetal kingdom the 
cell is the morphological unit, is a proposition admitting of a 
better defence, than the proposition that the cell is the mor- 
phological unit throughout the animal kingdom. The qualifi- 
cations with which, as we saw, the cell-doctrine must be 
taken, are qualifications thrust upon us more especially by 
the facts which zoologists have brought to light. It is 
among the Protozoa that there occur numerous cases of vital 
activity displayed by specks of protoplasm; and from the 
minute anatomy of all creatures above these, up to the Teleozoa, 
are drawn the numerous proofs that non-cellular tissues may 
arise by direct metamorphosis of structureless colloidal sub- 
stance. 

Our survey of morphological composition throughout the 
animal kingdom, must therefore begin with those undiffer- 
entiated aggregates of physiological units, out of which are 
formed what we call, with considerable license, morphological 
units. 


§ 200. In that division of the Protozoa distinguished as. 
Rhizopoda, are presented, under various modifications, these 
minute portions of living organic matter, so little differenta- 


78 MORPHOLOGICAL DEVELOPMENT. 


ated, if not positively undifferentiated, that animal individus 
ality can scarcely be claimed for them. Figs. 181, 182, and 


133, represent certain nearly-allied types of these—Ameba, 
Actinophrys, and Lveberkiihnia. The viscid jelly or sarcode, 
comparable in its physical properties to white of egg, out of 
which one of these creatures is mainly formed, shows us in 
various ways, the feebleness with which the component physio- 
logical units are integrated—shows us this by its very slight 
cohesion, by the extreme indefiniteness and mutability of its 
form, and by the absence of a limiting membrane. Though 
unqualified adherents of the cell-doctrine assert that the 
Amceba has an investment, yet since this investment, com- 
pared by Dujardin to the film which forms on the surface of 
paste, does not prevent the taking of. solid particles into the 
mass of the body, and does not, in such kindred forms as Fig. 
138, prevent the pseudopodia from coalescing when they 
meet, it cannot be anything deserving the name of a cell- 
wall. A considerable portion of the body, however, in Difflu- 
gia, Fig. 134, has a denser coating; so that the protrusion of 
the pseudopodia is limited to ‘one part of it. And in the 
solitary Foraminifera, like Gronuwa, the sarcode is covered 
over most of its surface by a delicate calcareous shell, pierced 
with minute holes, through which the slender pseudopodia 
are thrust. The Gregarina exhibits an advance in 
integration, and a consequent greater definiteness. Figs. 
135 and 186, exemplifying this type, show the complete 
membrane in which the substance of the creature is con- 
tained. Here there has arisen what may be properly called 
a cell: under its solitary form this animal is truly unicellular. 
Its embryology has considerable significance. After passing 
through a certain quiescent, ‘“ encysted”’ state, its interior 
breaks up into small portions, which, after their exit, assume 


THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 79 


forms like that of the Ameba; and from this young condi- 
tion in which they are undifferentiated, they pass into that 
adult condition in which they have limiting membranes. If 
this development of the individual Gregarina typifies the 
mode of evolution of the species, it yields further support to 
the belief, that homogeneous fragments of sarcode existed 
earlier than any of the structures which are properly called 
cells. Among aggregates of the first order, there 
are some much more highly developed. These are the Infu- 
soria; constituting the most numerous of the Protozoa, in 
species as in individuals. Figs. 137, 1388, and 139, are ex- 
amples. In them we find, along with greater definiteness, 
a considerable heterogeneity. The sarcode of which the body 
consists, has an indurated outer layer, bearing cilia and some- 
times spines; there is an opening serving as mouth, a per- 
manent cesophagus, and a cavity or cavities, temporarily 
formed in the interior of the sarcode, to serve as one or more 
stomachs ; and there is a comparatively specific arrangement 
of these and various minor parts. 

Thus in the animal kingdom, as in the vegetal kingdom, 
there exists a class of minute forms having this peculiarity, 
that no one of them is separable into a number of visible com- 
ponents homologous with one another—no one of them can 
be resolved into minor individualities. Its proximate units 
are those physiological units of which we conclude every or- 


ganism consists. The aggregate is an aggregate of the first 
order. | 


§ 201. Among plants are found types indicating a transi- 
tion from aggregates of the first order to aggregates of the 
second order; and among animals we find analogous types. 
But the stages of progressing integration are not here so dis- 
tinct. The reason probably is, that the simplest animals, 
having individualities much less marked than those of the 
simplest plants, do not afford us the same facilities for ob- 
servation. In proportion as the limits of the minor indi- 


80 MORPHOLOGICAL DEVELOPMENT. 


vidualities are indefinite, the formation of major individu- 
alities out of them, naturally leaves less conspicuous traces. 
Be this as it may, however, in such types of Protozoa as 
the Thalassicolle, we find that though there is reason to re- 
gard the aggregate as an aggregate of the second order, yet 
its divisibility into minor individualities like those just de- 
scribed, is by no means manifest. Fig. 140, representing 


Spherozoum punctatum, one of this group, illustrates the diffi- 
culty. Only by some license of interpretation, can we regard 
the ‘ celleeform bodies ”’ contained in it, as the morphological 
units of the animal. The jelly-like mass in which they are 
imbedded, shows no signs of being divisible into portions 
having each a cell or nucleus for its centre.* Comparison of 
the various forms assumed by creatures of this type, suggests, 
contrariwise, that the homogeneous sarcode is primary, and 
its included structures secondary. Among the 
Foraminifera, we find evidence of the coalescence of aggre- 
gates of the first order, into aggregates of the second order. 
There are solitary Foraminifers, allied to the creature repre- 
sented in Fig. 1384. Certain ideal types of combination 


* This statement seems at variance with the figure; but the figure is very in- 
accurate. Its inaccuracy curiously illustrates the vitiation of evidence. When I 
saw the drawing on the block, I pointed out to the draughtsman, that he had 
made the surrounding curves much more obviously related to the contained bodies, 
than they were in the original (in Dr Carpenter’s Foraminifera) ; and having 
looked on while he in great measure remedied this defect, thought no further care 
was needed. Now, however, on seeing the figure in the printer’s proof, I find 
that the engraver, swayed by the same supposition as the draughtsman that such 
a relation was meant to be shown, has made his lines represent it still more de- 
cidedly than those of the draughtsman before they were corrected. Thus, vague 
linear representations, like vague verbal ones, are apt to grow more definite 
when repeated. Hypothesis warps perceptions as it warps thoughts. 


THE MORPHOLOGICAL COMPOSITION OF ANIMALS. Sl 


among them, are shown in Fig. 141 And setting out from 
these, we may ascend in various directions to kinds com- 
pounded to an immense variety of degrees in an immense 
variety of ways. In all of them, however, the separability of 
the major individuality into minor individualities, is very in- 
complete. ‘The portion of sarcode contained in one of these 
calcareous chambers, gives origin to an external bud; and 
this presently becomes covered, like its parent, with calcareous 
matter: the position in which each successive chamber is so 
produced, determining the form of the compound shell. But 
the portions of sarcode thus budded out one from another, do 
not become distinctly individualized. Fig. 142, representing 
the living net-work which remains when the shell of an Or- 
bitolite has been dissolved, shows the continuity that exists 
among the occupants of its aggregated chambers. Still, the 
occupant of each chamber may fairly be considered as homo- 
logous with a solitary Foraminifer; and if so, the Orbitolite 
is an aggregate of the second order: this indefinite marking- 
off of its morphological units, being the obverse of the fact 
that the individualities of their prototypes are feebly pro- 
nounced. Forms of essentially the same kind 
are ageregated in another manner among the Spongide. 
The fibres of a living sponge are clothed with gelatinous 
substance, which is separable into Ameba-like creatures, 
capable of moving about by their pseudopodia when detach- 
ed. These nucleated portions of sarcode, which are the 
morphological units of the sponge, lining all its channels 
and chambers, subsist on the nutritive particles brought to 
them by the currents of water that are drawn in through 
the superficial pores, and sent out through the larger open- 
ings—currents produced by ciliated units, such as are shown 
in Fig. 148. So that, in the words of Prof. Huxley, “the 
sponge represents a kind of subaqueous city, where the people 
are arranged about the streets and roads, in such a manner, 
that each can easily appropriate his food from the water as 1t 


passes along.” In the compound Infusoria, the 
VOL, II. 6 


82 MORPHOLOGICAL DEVELOPMENT. 


component units remain quite distinct. Being, as aggre- 
gates of the first order, much more definitely organized, 
their union into aggregates of the second order does not de- 
stroy their original individualities. Among the Vorticelle, 
of which two kinds are delineated in Figs. 144 and 145, there 
are various illustrations of this: the members of the com- 
munity being sometimes appended to a single stem; some- 
times attached by long separate stems to a common base ; and 
sometimes massed together. 

Thus far, these aggregates of the second order exhibit but 
indefinite individualities. The integration is physical; but 
not physiological. Though, in the Thalassicolle, there is a 
shape that has some symmetry; and though, in the Fora- 
minifera, the formation of successive chambers proceeds in such 
methodic ways, as to produce quite-regular and tolerably-spe- 
cific shells; yet no more in these than in the Sponges or the 
compound Vorticelle, do we find such co-ordination as gives 
the whole a life predominating over the lives of its parts. 
We have not yet reached an aggregate of the second order, 
so individuated as to be capable of serving as a unit in still 

higher combinations. But in 
vty __ the class Cclenterata, this ad- 


= y) 23 vance is displayed. The com- 
0, UB BS 5 mon Hydra, habitually taken as 


OS He the type of the lowest division 


9 

Oy" 2) of this class, has specialized 
parts performing mutually-subservient functions; and thus 
exhibiting a total life distinct from the lives of the units. 
Fig. 146 represents one of these creatures in its contracted 
state and in its expanded state; while Fig. 147 is a 
rude diagram from memory showing the wall of this 
creature’s sack-like body as seen in section under the 
microscope: @ and b being the outer and inner cellular 
layers; while in the central space between them, is 
that nucleated: substance, or sarcode, or protoplasm, 
in which the cells originate. But this lowly-organized 


THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 83. 


tissue of the Hydra, illustrates a phase of integration in 
which the lives of the minor aggregates are only par: 
tially-subordinated to the life of the major aggregate 
formed by them. Tor a Hydra’s substance is separable into 
Ameeba-like portions, capable of moving about independ- 
ently. Prof. Green quotes Ecker, Lewes, and Jiger, in proof 
that “this animal exhibits, at certain seasons of the year, a 
tendency to break up into particles of a sarcode aspect, which 
retain for a long time an independent vitality.” And if we 
bear in mind how analogous are the extreme extensibility 
and contractility of a Hydra’s body and tentacles, to the pro- 
perties displayed by the sarcode among Rhizopods; we may 
infer that probably the movements and other actions of a 
Hydra, are due to the half-independent co-operation of the 
Ameba-like individuals composing it. 


§ 202. A truth which we before saw among plants, we 
here see repeated among animals—the truth that as soon as 
the integration of aggregates of the frst order into aggregates 
of the second Aah produces compound wholes so specific in 
their shapes and sizes, and so mutually dependent in their 
parts, as to have distinct individualities ; there simultaneously 
arises the tendency in them to produce, by gemmation, other 
such aggregates of the second order. The approach towards 
definite limitation in an organism, is, by implication, an ap- 
proach towards a state in which growth passing a certain point, 
results, not in the increase of the old individual, but in the 
formation of a new indi- 
vidual. Thus it happens 
that the common polype 
buds out other polypes, 
some of which very shortly | 
do the like, as shown in 
Fig. 148: a process paral- 
leled by the fronds of sundry He ae by those of the lower 


Jungermanniacee. And just as, among these last plants, the 
6 * 


84 MORPHOLOGICAL DEVELOPMENT. 


proliferously-produced fronds, after growing to certain sizes 
and developing rootlets, detach themselves from their parent- 
fronds; so among these animals, separation of the young 
ones from the bodies of their parents, ensues when they have 
acquired tolerably complete organizations. 

There is reason to think that the parallel holds still fur- 
ther. Within the limits of the Jungermanniacee, we found 
that while some genera exhibit this discontinuous develop- 
ment, other genera exhibit a development that is similar to 
it in all essential respects, save that it is continuous. And 
here within the limits of the Hydrozoa, we find, along with 
this genus in which the gemmiparous individuals are pre- 
sently cast off, other genera in which they are not cast off, but 
form a permanent aggregate of the third order. Figs. 149 
and 150, exemplify these compound Hydrozoa—one of them 
showing this mode of growth so carried out as to produce a 
single axis; and the other showing how, by repetitions of 
the process, lateral axes are produced. Integrations character- 
izing certain higher genera of the Hydrozoa, which swim or 
float instead of being fixed, are indicated by Figs. 151 and 
152: the first of them representing the type of a group in 
which the polypes growing from an 
axis, or ccenosare, aredrawn through the 
water by the rhythmical contractions 
of the organs from which they hang ; 
and the second of them representing 
a Physalia the component polypes 
of which are united into a cluster, 
attached to an air-vessel. It should 
be added that in the Rhizostomes, the 
integration is carried so far, that the 
individualities of the polypes are al- 
most lost in that of the aggregate 
they form. 

A parallel series of illustrations 
might be drawn from that second di- 


THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 89 


vision of the Celenterata, known as the Actinozoa. Here, too, 
we have a group of species—the Sea-anemonies—the individ- 
uals of which are solitary. Here, too, we have agamogenetic 
multiplication: occasionally by gemmation, but more fre- 
quently by that modified process called spontaneous fission. 
And here, too, we have compound forms resulting from the 
arrest of this spontaneous fission before it is complete. To 
give examples is needless; since they would but show, in 
more varied ways, the truth already made sufficiently clear, 
that the compound Celenterata are aggregates of the third 
order, produced by integration of aggregates of the second 
order such as we have in the Hydra. As before, it is 
manifest that on the hypothesis of evolution, these higher in- 
tegrations will insensibly arise, if the separation of the gem- 
miparous polypes is longer and longer postponed ; and that an 
increasing postponement will result by survival of the fittest, 
if it profits the group of individuals to remain united instead 
of dispersing. 


§ 203. The like relations exist, and mmply that the like 
processes have been gone through, among those more highly- 
organized animals called Molluscoida. We have solitary 
individuals, and we have variously-integrated groups of indi- 
viduals: the chief difference between the evidence here fur- 
nished, and that furnished in the last case, being the absence 
of a type obviously linking the solitary state with the aggre- 
gated state. 

Itis now an accepted belief that the creatures named Brachi- 
opoda, very abundant in the so-called palzeozoic times, but at 
present comparatively rare, are akin in structure to the 
Polyzoa ; widely as they differ from them in size. If we can- 
not fairly say that by union of many Brachiopods there would 
be produced a compound animal like a Polyzoon; yet we may 
fairly say that were a small imperfectly-developed Brachiopod 
united with others like itself, a Polyzoon would result. This in- 
tegration of aggregatesof the second order, is carried on among 


86 MORPHOLOGICAL DEVELOPMENT. 


the Polyzoa in divers ways, and with different degrees of com- 
pleteness. Thelittle patches of minute cells, shownas magnified 
in Fig. 153, so common on the fronds of sea-weeds and the 
surfaces of rocks at low-water mark, display little beyond me- 
chanical combination. The adjacent individuals, though sever- 
ally originated by gemmation from the same germ, have but 
little physiological dependence. In kindred kinds, however, 
as shown in Figs. 154 and 155, one of which is a magnified 
portion of the other, the integration is somewhat greater: 
the co-operation of the united individuals being shown in 
the production of those tubular branches which form their 


) 


USE 4355 


common support, and establish among them a more decided 
community of nutrition. 

Among the Ascidians, another order of the Molluscoida, this 
general law of morphological composition is once more dis- 
played. Each of these creatures subsists on the nutritive 
particles contained in the water which it draws in through 
one orifice and sends out through another; and it may thus 
subsist either alone, or in connexion with others that are 
in some cases loosely aggregated and in other cases closely 
aggregated. Fig. 156, Phallusia mentula, is one of the soli- 


SET 


tary forms. A type in which the individuals are united by a 
stolon that gives origin to them by successive buds, is shown 
in Perophora, Fig. 157. Among the Botryllide, of which one 


THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 87 


kind is drawn on a small scale in Fig. 159, and a portion of 
the same on a larger scale in Fig. 158, there is a combination 
of the individuals into annular clusters, which are themselves 
imbedded in a common gelatinous matrix. And in this 
group there are integrations even a stage higher, in which 
several such clusters of clusters grow from a single base. 
Here the compounding and re-compounding, appears to 
be carried further than anywhere else in the animal 
kinedom. 

Thus far, however, among these aggregates of the third 
order, we see what we before saw among the simpler aggre- 
gates of the second order—we see that the component indi- 
vidualities are but to a very small extent subordinated to the 
individuality made up of them. In nearly all the forms in- 
dicated, the mutual dependence of the united animals is so 
slight, that they are more fitly comparable to societies, of 
which the members co-operate in securing certain common 
benefits. There is scarcely any specialization of functions 
among them. Only in the last tyne described do we see a 
number of individuals so completely combined as to simulate 
a single individual. And even here, though there appears to 
be an intimate community of nutrition, there is no physio- 
logical integration beyond that implied in several mouths and 
stomachs having a common vent. 


§ 204. We come now to an extremely interesting ques- 
tion. Does there exist in other sub-kingdoms composition of 
the third degree, analogous to that which we have found so 
prevalent among the Ce/enterata and the Molluscoida ? The 
question is not whether elsewhere there are tertiary aggregates 
produced by the branching or clustering of secondary aggre- 
gates, in ways like those above traced ; but whether elsewhere 
there are aggregates which, though otherwise unlike in the 
arrangement of their parts, nevertheless consist of parts so 
similar to one another that we may suspect them to be 
united secondary aggregates. The various compound types 


88 MORPHOLOGICAL DEVELOPMENT. 


above described, in which the united animals maintain their 
individualities so distinctly that the individuality of the 
aggregate remains vague, are constructed in such ways that 
the united animals carry on their several activities with 
scarcely any mutual hindrance. The members of a branched 
Hydrozoon such as is shown in Fig. 149 or Fig. 150, are so 
placed that they can all spread their tentacles and catch 
their prey as well as though separately attached to stones or 
weeds. Packed side by side on a flat surface or forming a tree- 
like assemblage, the associated individuals among the Polyzoa 
are not unequally conditioned; or if one has some advantage 
over another in a particular case, the mode of growth and 
the relations to surrounding objects are so irregular as to 
prevent this advantage re-appearing with constancy in suc- 
cessive generations. Similarly with the Ascidians growing 
from a stolon or those forming an annular cluster: each of 
them is as well placed as every other for drawing in the 
currents of sea-water from which it selects its food. In 
these cases the mode of aggregation does not expose the 
united individuals to multiform circumstances; and there- 
fore is not calculated to produce among them any structural 
multiformity. or the same reason no marked physiologi- 
cal division of labour arises among them; and consequently 
no combination close enough to disguise their several indi- 
vidualities. But under converse conditions we may expect 
converse results. If there isa mode of integration which 
necessarily subjects the united individuals to unlike sets of 
incident forces, and does this with complete uniformity from 
generation to generation, it 1s to be inferred that the united 
individuals will become unlike. They will severally assume 
such different functions as their different positions enable 
them respectively to carry on with the greatest advantage to 
the assemblage. This heterogeneity of function arising 
among them, will be followed by heterogeneity of structure ; 
as also by that closer combination which the better enables 
them to utilize one another’s functions. And hence, while 


THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 89 


the originally-like individuals are rendered unlike, they will 
have their homologies further obscured by their progressing 
fusion into an aggregate individual of a higher order. 

These converse conditions are in nearly all cases fulfilled 
where the successive individuals arising by continuous devel- 
opment are so budded-off as to form a linear series. I say 
in nearly all cases, because there are some types in which 
the associated individuals, though joined in single file, are 
not thereby rendered very unlike in their relations to the 
environment ; and therefore do not become differentiated and 
integrated to any considerable extent. I refer to such Asci- 
dians as the Salpide. These creatures float passively in the 
sea, attached together in strings. Being placed side by side 
and having mouths and vents that open laterally, each of 
them is as well circumstanced as its neighbours for absorb- 
ing and emitting the surrounding water; nor have the in- 
dividuals at the two extremities any marked advantages 
over the rest in these respects. Hence in this type, and in 
the allied type Pyrosoma, which has its component indivi- 
duals built into a hollow cylinder, linear aggregation may 
exist without the minor individualities becoming obscured 
and the major individuality marked: the conditions under 
which a differentiation and integration of the component 
individuals may be expected, are not fulfilled. But where 
the chain of individuals produced by gemmation, is either 
habitually fixed to some solid body by one of its extremities 
or moves actively through the water or over submerged 
stones and weeds, the several members of the chain become 
differently conditioned in the way above described ; and may 
therefore be expected to become unlike while they become 
united. A clear idea of the contrast between these two 
linear arrangements and their two diverse results, will be 
obtained by considering what happens to a row of soldiers, 
when changed from the ordinary position of a single rank 
to the position of Indian file. So long as the men stand 
shoulder to shoulder, they are severally able to use their 


90 MORPHOLOGICAL DEVELOPMENT. 


weapons in like ways with like efficiency; and could, if: 
called on, similarly perform various manual processes directly 
or indirectly conducive to their welfare. But when on the 
word of command “right face,” they so place themselves 
that each has one of his neighbours before him and another 
behind him, nearly all of them become incapacitated for 
fighting and for many other actions. They can walk or run 
one after another, so as to produce movement of the file in 
the direction of its length; but if the file has to oppose an 
enemy or remove an obstacle lying in the line of its march, 
the front man is the only one able to use his weapons or 
hands to much purpose. And manifestly such an arrange- 
ment could become advantageous only if the front man pos- 
sessed powers peculiarly adapted to his position, while those 
behind him facilitated his actions by carrying supplies, &e. 
This simile, grotesque as it seems, serves to convey better 
perhaps than any other could do, a clear idea of the relations 
that must arise in a chain of individuals arising by gemma- 
tion, and continuing permanently united end to end. Such 
a chain can arise by natural selection, only on condition that 
combination is more advantageous than separation ; and for 
it to be more advantageous, the anterior members of the series 
must become adapted to functions facilitated by their posi- 
tions, while the posterior members become adapted to func- 
tions which their positions permit. Hence, survival of the 
fittest must tend continually to establish types in which the 
connected individuals are more unlike one another, at the 
same time that their several individualities are more dis- 
guised by the integration consequent on their mutual 
dependence. 

Such being the anticipations warranted by the general 
laws of evolution, we have now to inquire whether there 
are any animals which fulfil them. Very little search 
suffices ; for structures of the kind to be expected are abund- 
ant. In that great division of the animal kingdom called 
Annulosa, especially if the Annuloida be regarded as part of 


THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 9] 


it, we find a variety of types having the looked-for charac- 
ters. Let us contemplate some of them. 


§ 205. An adult Annelid is composed of segments which 
repeat one another in their details as well as in their general 
shapes. Dissecting one of the lower orders, such as is 
shown in Fig. 160, proves that the successive segments, be- 


sides having like locomotive appendages, like branchie, and 
sometimes even like pairs of eyes, also have like internal 
organs. Each has its enlargement of the alimentary canal ; 
each its contractile dilatation of the great blood-vessel ; each 
its portion of the double nervous cord, with ganglia when 
these exist; each its branches from the nervous and vascular 
trunks answering to those of its neighbours; each its simi- 
larly answering set of muscles; each its pair of openings 
through the body-wall; and so on throughout, even to the 
organs of reproduction. That is to say, every segment is in 
great measure a physiological whole—every segment con- 
tains most of the organs essential to individual life and mul- 
tiplication: such essential organs as it does not contain, 
being those which its position as one in the midst of a chain, 
prevents it from having or needing. If we 
ask what is the meaning of these homologies, no adequate 
answer is supplied by any current hypothesis. That this 
“vegetative repetition’ is carried out to fulfil a prede- 
termined plan, was shown to be quite an untenable notion 
(§§ 133, 184). On the one hand, we found nothing satis- 
factory in the conception of a Creator who prescribed to him- 


92 MORPHOLOGICAL DEVELOPMENT. 


self a certain unit of composition for all creatures of a par- 
ticular class, and then displayed his ingenuity in building up 
a great variety of forms without departing from the “ arche- 
typal idea.” On the other hand, examination made it mani- 
fest that even were such a conception worthy of being enter- 
tained, it would have to be relinquished ; since in each class 
there are numerous deviations from the supposed “ archetypal 
idea.’ Still less can these traits of structure be accounted 
for teleologically. That certain organs of nutrition and re- 
spiration and locomotion are repeated in each segment of a 
dorsibranchiate annelid, may be regarded as functionally ad- 
vantageous for a creature following its mode of life. But 
why should there be a hundred or even two hundred pairs of 
ovaries? This is an arrangement at variance with that 
physiological division of labour which every organism pro- 
fits by—is a less advantageous arrangement than might have 
been adopted. That is to say, the hypothesis of a designed 
adaptation fails to explain the facts. Contrariwise, 
these structural traits are just such as might naturally be 
looked for, if these annulose forms have arisen by the in- 
tegration of simpler forms. Among the various compound 
animals already glanced at, it is very general for the united 
individuals to repeat one another in all their parts—repro- 
ductive organs included. Hence if, instead of a clustered or 
branched integration, such as the Cedenterata and Molluscoida 
exhibit, there occurs a longitudinal integration; we may ex- 
pect that the united individuals will habitually indicate their 
original independence by severally bearing germ-producing 
or sperm-producing organs. 

The reasons for believing one of these creatures to be an 
ageregate of the third order, are greatly strengthened when 
we turn from the adult structure to the mode of develop- 
ment. Among the Dorsibranchiata and Tubicole, the em- 
bryo leaves the egg in the shape of a ciliated gemmule, not 
much more differentiated than that. of a polype. As shown 
in Fig. 162, it is a nearly globular mass; and its interior 


THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 93 


consists of untransformed cells. The first appreciable change 
is an elongation and a simultaneous commencement of seg- 
mentation. The segments multiply by a modified gemma- 
tion, which takes place from the hinder end of the penultimate 
segment. And considerable progress in marking out these 
divisions is made before the internal organization begins, 
Figs. 163, 164, 165, represent some of t 


SS 


162 . 
163 


65 


Annelids of other orders, the embryo assumes the segmented 
form while still in the egg. But it does this in just the 
same manner as before. Indeed, the essential identity of the 
two modes of development is shown by the fact that the seg- 
mentation within the egg is only partially carried out: in 
all these types the segments continue to increase in number 
for some time after birth. Now this process is as 
like that by which compound animals in general are formed, 
as the different conditions of the case permit. When new 
individuals are budded-out laterally, their unfolding is not 
hindered—there is nothing to disguise either the process or 
the product. But gemme produced one from another in the 
same straight line, and remaining connected, restrict one 
another’s developments ; and that the resulting segments are 
so many gemmiparously-produced individuals, is necessarily 
less obvious. 


§ 206. Evidence remains which adds very greatly to the 
weight of that already assigned. Thus far we have studied 
only the individual annulose animal ; considering what may 
be inferred from its mode of evolution and final organization. 


94 MORPHOLOGICAL DEVELOPMENT. 


We have now to study annulose animals in general. Com; 
parison of them will disclose various phases of progressive 
integration of the kind to be anticipated. 

Among the simpler Annuloida, as in the Nemertide and in 
some kinds of Planaria, transverse fission occurs. A por- 
tion of a Planaria separated by spontaneous constriction, 
becomes an independent individual. Sir J. G. Dalyell found 
that in some cases numerous fragments artificially separated, 
grew into perfect animals. In these annuloids which thus 
remind us of the lowest Hydrozoa in their powers of agamo- 
genetic multiplication, the individuals produced one from 
another, do not continue connected. As the young ones 
laterally budded-off by the Hydra separate when complete, 
so do the young ones longitudinally budded-off by the Pia- 
naria. Fig. 166 indicates this. But there are allied types 
which show us a more or less persistent union of homologous 
parts, or individuals, similarly arising by longitudinal gem- 
mation. The cestoid Hntozoa furnish illustrations. Without 
dwelling on the fact that each segment of a Tena, like each 
separate Planaria, is an independent hermaphrodite, or on the . 
fact that both develop their ova by the peculiar method of 
forming germinal vesicles in one canal and surrounding them 
with yelk that is secreted in another canal; and without 
specifying the sundry common structural traits which add 
probability to the suspicion that there is some kinship be- 
tween the individuals of the one order and the segments of 
the other; it will suffice to point out that the two types are 
so far allied as to demand their union under the same sub- 
class title. And recognizing this kinship, we see significance 
in the fact that in the one case the longitudinally-produced 
gemme separate as complete individuals, and in the other 
continue united as segments in smaller or larger numbers 
and for shorter or longer periods. In Tenia echinococcus, 
represented in Fig. 167, we have a species in which the 
number of segments thus united does not exceed four. In 
Echinobothrium typus there are eight or ten; and in cestoids 


THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 95 


generally they are numerous.* A considerable 
hiatus occurs between this phase of integration and the next 
higher phase which we meet with; but it is not greater 
than the hiatus between the types of the Annuloida and the 
Annelida, which present the two phases. Though it is 
doubtful whether separation of single segments occurs among 
the Annelida, yet very often we find strings of segments, 
arising by repeated longitudinal budding, which after reach- 
ing certain lengths undergo spontaneous fission: in some 
cases doing this so as to form two or more similar strings 
of segments constituting independent individuals; and in 
other cases doing it so that the segments spontaneously 
separated are but a small part of the string. Thus a Syllis, 
Fig. 168, after reaching a certain length, begins to trans- 


form itself into two individuals: one of the posterior seg- 
ments develops into a head, and simultaneously narrows its 
connexion with the preceding segments, from which it 


* T find that the reasons for regarding the segment of a Tenia as answering 
to an individual of the second order of aggregation, are much stronger than I sup- 
posed when writing the above. Van Beneden says:—‘“ Le Proglottis (segment) 
ayant acquis tout son développement, se détache ordinairement de la colonie et 
continue encore & croitre dans Vintestin du méme animal; il change méme sou- 
vent de forme et semble doué d’une nouvelle vie ; ses angles s’effacent, tout le corps 
s’arrondit, et il nage comme une Planaire au milieu des muscosités intestinales,” 


96 MORPHOLOGICAL DEVELOPMENT. 


eventually separates. Still more remarkable is the extent to 
which this process is carried in certain kindred types; which 
exhibit to us several individuals thus being simultaneously 
formed out of groups of segments. Fig. 169, copied (omit- 
ting the appendages) from one contained in a memoir 
by M. Milne-Edwards, represents six worms of different 
ages in course of development: the terminal one being the 
eldest, the one having the greatest number of segments, 
and the one that will first detach itself; and the success- 
ively anterior ones, with their successively smaller numbers 
of segments, being successively less advanced towards fitness 
for separation and independence. Here among groups of 
segments we see repeated what in the previous cases occurs 
with single segments. And then in other Annelids we find that 
the string of segments arising by gemmation from a single 
germ becomes a permanently united whole: the tendency to 
any more complete fission than that which marks out the seg- 
ments, being lost; or, in other words, the integration having 
become relatively complete. Leaving out of sight the 
question of alliance among the types above grouped together, 
that which it here concerns us to notice is, that longitudinal 
gemmation does go on; that it is displayed in that primitive 
form in which the gemme separate as soon as produced ; that 
we have types in which such gemme hang together in 
groups of four, or in groups of eight and ten, from which 
however the gemme successively separate as individuals; 
that among higher types we have long strings of similarly- 
formed gemme which do not become individually independ- 
ent, but separate into organized groups; and that from 
these we advance to forms in which all the gemmez remain 
parts of a single individual. One other significant 
class of facts must be added. A few cases have been pointed 
out, one of them quite recently, in which Annelids mul- 
tiply by lateral gemmation. M. Pagenstecher alleges this 
of the Exogone gemmifera: describing a certain number of 
the segments of the body as severally bearing on their dorsal 


THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 97 


surfaces a bud on each side. And M. L. Vaillant, after 
citing this observation of M. Pagenstecher, gives an account 
of a species of Sy/lis in which a great number of buds were 
borne by asingle segment. That the longitudinally-produced 
gemme which compose an Annelid, should thus have, one of 
them or several of them, the power of laterally budding-off 
gemme, from which no doubt other annelids arise, oives fur- 
ther support to the hypothesis that, primordially, the seg- 
ments were independent individuals. And it suggests this be- 
hef the more strongly because, in certain types of Coelenterata, 
we see that longitudinal and lateral gemmation do occur to- 
gether, where the longitudinally-united gemme are demon- - 
strably independent individuals. 


§ 207. It would add to the probability of this conclusion 
could we identify the type out of which the annulose type 
may have arisen by the process of integration. I believe 
there may be pointed out such a type—a type which, by a 
slight modification carrying somewhat further an habitual 
mode of development, would produce not only a unit of com- 
position for the annulose type, but also as a bond uniting it 
with the other types, and these with one another. It is un- 
desirable, however, here to enter upon the numerous explan- 
ations involved by opening the question of these relation- 
ships ; both because it would necessitate a long digression, 
encumbering too much the general argument, and because, 
being highly speculative, it would be impolitic to let the 
general argument be even apparently implicated by it. 

But even supposing it impossible now to identify the unit of 
composition of the annulose type, the foregoing evidence still 
goes far towards showing that an annulose animal is an agere- 
gate of the third order. This repetition of segments, some- 
times numbering several hundreds, like one another in all 
their organs even down to those of reproduction, while it is 
otherwise unaccountable, is fully accounted for if these seg- 


ments are homologous with the separate individuals of some 
VOL, II. 7 


98 MORPHOLOGICAL DEVELOPMENT. 


lower type. The gemmation by which these segments are pro 
duced, is as similar as the conditions allow, to the gemmation 
by which compound animals in general are produced. As 
among plants and as among demonstrably-compound animals, 
we see that the only thing required for the formation of a per- 
manent chain of gemmiparously-produced individuals, is that 
by remaining associated, such individuals will have advantages 
greater than are to be gained by separation. Further, by 
comparison of the annuloid and lower annulose forms, we 
discover a number of those transitional phases of integration 
which the hypothesis leads us to expect. And, lastly, the 
differences among these united individuals or successive 
segments, are not greater than the differences in their posi- 
tions and functions explain—not greater than such differences 
are known to produce among other united individuals: wit- 
ness sundry compound LHydrozoa. 
Indirect evidence of much weight has still, to be given. 
Thus far we have considered only the less-developed Annu- 
losa. ‘The more integrated and more differentiated types of 
the class remain. If in them we find a carrying further of 
the processes by which the lower types are here supposed to 
have been evolved, we shall have additional reason for be- 
lieving them to have been so evolved. If we find that in 
these superior orders the individualities of the united seg- 
ments are much less pronounced than in the inferior, we 
shall have grounds for suspecting that in the inferior the 
individualities of the segments are less pronounced than in 
those lost forms which initiated the annulose sub-kingdom. 


CHAPTER V. 


THE MORPHOLOGICAL COMPOSITION OF ANIMALS, 
CONTINUED. 


§ 208. Insects, Arachnids, Crustaceans, and Myriapods, 
are all members of that higher division of the Annu/losa called 
Articulata or Arthropoda. ‘Though in these creatures the 
formation of segments may be interpreted as a disguised 
gemmation ; and though in some of them the number of seg- 
ments increases by this modified budding after leaving the 
egg, as among the higher Annelids; yet the process is not 
nearly so dominant: the segments are usually much less 
numerous than we find them in the types last considered. 
In most cases, too, the segments are in a greater degree dif- 
ferentiated one from another, at the same time that they 
are severally more differentiated within themselves. Nor is 
there any instance of spontaneous fission taking place in the 
series of segments composing an articulate animal. On the 
contrary, the integration, always great enough permanently 
to unite the segments, is frequently carried so far as to hide 
very completely the individualities of some or many of them ; 
and occasionally, as among the Acari, the consolidation, or 
the arrest of segmentation, is so decided as to leave scarcely 
a trace of the articulate structure: the type being in these 
cases indicated chiefly by the presence of those character- 
istically-formed limbs, which give the alternative name 
Arthropoda to all the higher Annulosa. Omitting the para- 
sitic orders, which, as in other cases, are aberrant members of 

7 * 


100 MORPHOLOGICAL DEVELOPMENT. 


their sub-kingdom, comparisons between the different orders 
prove that the higher are strongly distinguished from the 
lower, by the much greater degree in which the individual- 
ity of the tertiary aggregate dominates over the individual- 
ities of those secondary aggregates called segments or 
“ somites,” of which it 1s composed. The successive Figs. 
170—176, representing (without their limbs) a Julus, a 


Scolopendra, an isopodous Crustacean, and four kinds of 
decapodous Crustaceans, ending with a Crab, will convey at a 
glance an idea of the way in which that greater size and 
heterogeneity reached by the higher types, 1s accompanied 
by an integration which, in the extreme cases, almost obliter- 
ates all traces of composite structure. In the Crab the 
posterior segments, usually folded underneath the shell, 
alone preserve their primitive distinctness: so completely 
confluent are the rest, that it seems absurd to say that a 
Crab’s carapace is composed of as many segments as there are 
pairs of limbs, foot-jaws, and antennz attached to it; and 
were it not that during early stages of the Crab’s develop- 
ment the segmentation is faintly marked, the assertion might 
be considered illegitimate. 

That all articulate animals are thus composed from end to 
end of homologous segments, is, however, an accepted doc- 
trine among naturalists. It is a doctrine that rests on care- 


THE MORPHOLOGICAL COMPOSITION OF ANIMAIS. 101 


ful observation of three classes of facts—the correspondences 
of parts in the successive “somites” of an adult articulate 
animal; the still more marked correspondences of such 
parts as they exist in the embryonic or larval articulate ani- 
mal; and the maintenance of such correspondences in some 
types, which are absent in types otherwise near akin to them. 
The nature of the conclusion which these evidences unite in 
supporting, will best be shown by the annexed copies from 
the lecture-diagrams of Prof. Huxley; exhibiting the 
typical structures of a Myriapod, an Insect, a Spider, and a 
Crustacean, with their relations to a common plan, as in- 
terpreted by him. 


ie Insetl Shider 179 


iS) 


UDIIMJSUAD & 


Vii Sceeomaaeaaasas | 


Annelide 


Treating of these homologies, Prof. Huxley says “that a 
striking uniformity of composition is to be found in the heads 
of, at any rate, the more highly organized members of these 
four classes, and that, typically, the head of a Crustacean, 
an Arachnid, a Myriapod, or an Insect, is composed of six 
somites (or segments corresponding with those of the body) 
and their appendages, the latter being modified so as to 
serve the purpose of sensory and manducatory organs.” 
And omitting the Myriapods, he also finds among these 
groups the further unity that in most of them the entire 
animal contains twenty of these homologous segments. 


102 MORPHOLOGICAL DEVELOPMENT. 


Thus even in the higher Annuiosa, the much greater conso- 
lidation and much greater heterogeneity do not obliterate the 
evidence of the fact, that the organism is an aggregate of 
the third order. Beyond all question it is divisible into a 
number of proximate units, each of which has essentially the 
same structure as its neighbours, and each of which is an 
ageregate of the second order, in so far as it is an organized 
combination of those aggregates of the first order which we 
eall morphological units or cells. And that these segments 
or somites, which make up an annulose animal, were origin- 
ally aggregates of the second order having independent in- 
dividualities, is an hypothesis which gathers further support 
from the contrast between the higher and the lower articu- 
late types, as well as from the contrast between the Articu- 
lata in general and* the inferior Annulosa. For if that 
masking of the individualities ofthe segments which we find 
distinguishes the higher forms from the lower, has been going 
on from the beginning, as we may fairly assume ; it is to be 
inferred that the individualities of the segments in the lower 
forms, were originally more marked than they now are. 
Reversing those processes of change by which the most 
developed Annulosa have arisen from the least developed ; 
and applying in thought this reversed process to: the least 
developed, as they were described in the last Chapter; we 
are brought to the conception of attached segments that are 
completely alike, and have their individualities in no ap- 
preciable degree subordinated to that of the chain they com- 
pose. From which there is but a step to the conception of 
gemmiparously-produced individuals which severally part 
one from another as soon as they are formed. 


§ 209. We must now return to a point whence we di- 
verged some time ago. As before explained under the head 
of Classification, organisms do not admit of uni-serial ar- 
rangement, either in general or in detail; but everywhere 
form groups within groups. Hence, having traced the 


THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 1038 


phases of morphological composition up to the highest forms 
in any sub-kingdom, we find ourselves at the extremity of a 
great branch, from which there is no access to another great 
branch, except by going back to some place of bifurcation low 
down in the tree. 

The nearest relatives of the Mollusca are those molluscoid 
forms treated of early in the last Chapter. A Brachiopod or 
a solitary Ascidian, though widely unlike a Mussel or a 
Snail or a Cuttle-fish, is nearer akin to them than is any 
ccelenterate animal or annulose animal or vertebrate animal. 
One of the leading distinctions, however, between the Mol- 
luscoida and the Mollusca, considered as groups, is that 
whereas the Molluscowda are very frequently, or indeed 
generally, compound, the Mollusca are invariably single. 
No true Mollusk multiples by gemmation, either continuous 
or discontinuous ; but the product of every fertilized germ is 
a single individual. 

It is a significant fact that here, where for the first time 
we have homogenesis. holding throughout an entire sub- 
kingdom, we have also throughout an entire sub-king- 
dom no case in which the organism is divisible into two, 
three, or more, like parts. There is neither any such 
clustering or branching as a celenterate or molluscoid ani- 
mal usually displays; nor is there any trace of that seg- 
mentation which characterizes the Annulosa. Among these 
animals in which no single egg produces several individuals, 
no individual is separable into several homologous divisions. 
This connexion will be seen to have a probable meaning, on 
remembering that it is the converse of the connexion which 
obtains among the Annulosa, considered as a group. 

A Mollusk, then, is an aggregate of the second order. Not 
only in the adult animal is there no sign of a multiplicity of 
like parts that have become obscured by integration; but 
there is no sign of such multiplicity in the embryo. And 
this unity is just as conspicuous in the lowest Lamelli- 
branch as in the highest Cephalopod. 


104. MORPHOLOGICAL DEVELOPMENT. 


It may be well to note, however, more especially because. 
it illustrates a danger of misinterpretation presently to be 
guarded against, that there are certain Mollusks which si- 
mulate the segmented structure. Externally a Chiton, Fig. 
188, appears to be made up 
of divisions substantially like 
\/ those of the creature Fig. 
mr 189; and one who judged 
~* only by externals, would say 
that the creature Fig. 190 
differs as much from the 
creature Fig. 189, as this 
does from the preceding one. But the truth is, that while 
190 and 189 are closely-allied types, 189 differs from 188 
much more widely than a man does froma fish. And the 
radical distinction between them is this; that whereas in the 
Crustacean the segmentation is carried transversely through 
the whole mass of the body, so as to render the body more 
or less clearly divisible into a series of parts that are similarly 
composed ; in the Mollusk the segmentation is limited to the 
shell which it carries on its upper surface, and leaves its 
body as completely undivided as is that of a common slug. 
Were the body cut through at each of the divisions, the sec- 
tion of it attached to each portion of the shell would be unlike 
all the other sections. Here the segmentation has a purely 
functional derivation—is adaptive instead of genetic. The 
similarly-formed and similarly-placed parts, are not homolo- 
gous in the same sense as are the appendages of a pheenoga- 
mic axis or the limbs of an insect. 


Nw, 


182 


§ 210. In studying the remaining and highest sub-king- 
dom of animals, it is important to recognize this radical dif- 
ference in meaning between that likeness of parts which is 
produced by likeness of modifying forces, and that likeness 
of parts which is due to primordial identity of origin. On 
our recognition of this difference depends the view we take 


THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 105 


of certain doctrines that have long been dominant, and have 
still a wide currency. 

Among the Vertebrata, as among the Mollusca, homogenesis 
is universal. The two sub-kingdoms are like one another 
and unlike the remaining sub-kingdoms in this, that in 
all the types they severally include, a single fertilized ovum 
produces only a single individual. It is true that as the 
eggs of certain Gasteropods occasionally exhibit spontaneous 
fission of the vitelline mass, which may or mav no% result in 
the formation of two individuals; so among vertebrate ani- 
mals we now and then meet with double monsters, which 
appear to imply such a spontaneous fission imperfectly car- 
ried out. But these anomalies serve to render conspicu- 
ous the fact, that in both these sub-kingdoms the normal 
process is the integration of the whole germ-mass into a 
single organism, which at no phase of its development dis- 
plays any tendency to separate into two or more parts. 

Equally as throughout the Mollusca there holds throughout 
the Vertebrata, the correlative fact, that not even in its lowest 
any more than in its highest types, is the body divisible into 
homologous segments. The vertebrate animal, under its 
simplest as under its most complex form, is like the mollusc- 
ous animal in this, that you cannot cut it into transverse 
slices, each of which contains a digestive organ, a respiratory 
organ, a reproductive organ, &c. The organs of the. least- 
developed fish as well as those of the most-developed 
mammal, form but a single physiological whole; and they 
show not the remotest trace of having ever been divisible 
into two or more physiological wholes. That segmentation 
which the vertebrate animal usually exhibits throughout 
part of its organization, is the same in origin and meaning 
as the segmentation of a Chiton’s shell ; and no more implies 
in the vertebrate animal a composite structure, than do the 
successive pairs of branchiz of the Doto or the transverse rows 
of branchiz in the Holis, imply composite structure in the 
molluscous animal. To some this will seem a very question- 


206 MORPHOLOGICAL DEVELOPMENT. 


able proposition ; and had we no evidence beyond that which 
adult vertebrate animals of developed types supply, it would be 
a proposition not easy to substantiate. But abundant support 
for it is to be found in the structure of the vertebrate embryo, 
and in the comparative morphology of the Vertebrata in 
general. 

Embryologists teach us that the primordial relations of 
parts are most clearly displayed in the early stages of evo- 
lution; and that they generally become partially or com- 
pletely disguised in its later stages. Hence, were the verte- 
brate animal on the same level as the annulose animal in 
degree of composition—did it similarly consist of segments 
which are homologous in the sense that they are the prox- 
imate units of composition; we ought to find this funda- 
mental fact most strongly marked at the outset. As in 
the annelid-embryo, the first conspicuous change is the 
elongation and division into segments, by constrictions that 
encircle the whole body; and as in the articulate embryo, 
the blastoderm becomes marked out transversely into pieces 
which extend themselves round the yelk before the internal 
organization has made any appreciable progress; so in the 
embryo of every vertebrate animal, had it an analogous com- 
position, the first decided change should be a segmentation 
implicating the entire mass. But it is not so. Sundry im- 
portant differentiations occur before any divisions begin to 
show themselves. There is the defining of that elongated, 
elevated area with its longitudinal groove, which becomes the 
seat of subsequent changes; there is the formation of the 
notochord lying beneath this groove; there is the growth 
upwards of the boundaries of the groove into the dorsal 
lamine, which rapidly develop and fold over in the region of 
the head. Rathke, as quoted and indorsed by Prof. Huxley, 
describes the subsequent changes as follows :—“ The gelatin- 
ous investing mass, which, at first, seems only to constitute 
a band to the right and to the left of the notochord, forms 
around it, in the further course of development, a sheath, 


THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 107 


which ends in a point posteriorly. Anteriorly, it sends out 
two processes which underlie the lateral parts of the skull, 
but very soon coalesce for a longer or shorter distance. Pos- 
teriorly, the sheath projects but little beyond the notochord ; 
but, anteriorly, for a considerable distance, as far as the in- 
fundibulum. It sends upwards two plates, which embrace 
the future central parts of the nervous system laterally, pro- 
bably throughout their entire length.” All this precedes 
segmentation. Considered under its broadest aspects, the 
process is directly opposed to the process among the An- 
nulosa. Whereas among the Annulosa the first step is the 
resolution of the germ-mass or of the blastoderm into seg- 
ments, which may or may not afterwards become inte- 
grated; in the Vertebrata the first step is the marking 
out on the blastoderm of an integrated structure within 
which segments subsequently appear. When these do ap- 
pear, they are for some time limited to the middle region of 
the spinal axis; and nomore then than ever after, do they 
implicate the general mass of the body in their transverse di- 
visions. On. the contrary, before segmentation has made 
much: progress the rudiments of the vascular system are laid 
dowm in a manner showing not the remotest trace of any 
primordial correspondence of its parts with the divisions of the 
axis. No less at variance with the belief that the 
vertebrate animal is essentially a series of homologous parts, 
is the heterogeneity which exists among these parts on their 
first appearance. ‘Though in the head of an adult articulate 
animal there is little sign of divisibility into segments like 
those of the body; yet such segments, with their appropriate 
ganglia and appendages, are easily identifiable in the articu- 
late embryo. But in the vertebrata this antithesis is exactly 
reversed. At the time when segmentation has become de- 
cided in the dorsal region of the spine, there is no trace of 
segments in the parts that are to form the skull—nothin, 
whatever to suggest that the skull is being formed out of 
divisions homologous with vertebra. And minute observa- 


108 MORPHOLOGICAL DEVELOPMENT. 


tion no more discloses any such homology than does general, 
appearance. “ Remak,” says Prof. Huxley, “has more fully 
proved than any other observer, the segmentation into ‘ur- 
wirbel,’ or proto-vertebree, which is characteristic of the ver- 
tebral column, stops at the occipital margin of the skull— 
the base of which, before ossification, presents no trace of 
that segmentation which occurs throughout the vertebral 
column.” 

Consider next the evidence supplied by comparative mor- 
phology. In preceding sections (§§ 206, 208,) it has been 
shown that among annulose animals, the divisibility into 
homologous parts is most clearly demonstrable in the lowest 
types. Though in decapodous Crustaceans, in Insects, in 
Arachnids, there is difficulty in identifying some or many of 
the component somites; and though when identified they 
display only partial correspondences; yet on descending to 
Annelids, the composition of the entire body out of such 
somites becomes conspicuous, and the homology between each 
somite and its neighbours is shown by the repetition of one 
another’s structural details, as well as by their common 
gemmiparous origin: indeed, in some cases we have the 
homology directly demonstrated by seeing a somite of the 
body transformed into ahead. If, then, a vertebrate animal 
had a segmental composition of kindred nature, we ought to 
find it most clearly marked in the lowest Vertebrata, and 
most disguised in the highest Vertebrata. But here, as be- 
fore, the fact is just the reverse. Among the Vertebrata of 
developed type, such segmentation as really exists remains 
conspicuous—is but little obscured even in parts of the spinal 
column formed out of integrated vertebrae. Whereas in the 
undeveloped vertebrate type, segmentation is scarcely at all 
traceable. The Amphioxus, Fig. 191, is not only without 
ossified vertebrae; not only is it without cartilaginous re- 
presentatives of them; but it is even without anything like 
distinct membranous divisions. The spinal column exists 
as a continuous notochord: the only signs of incipient seg- 


THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 109 


mentation being given by its membranous sheath, in the 
upper part of which ‘“‘quadrate masses of somewhat denser 


tissue seem faintly to represent neural spines.” Moreover, 
throughout sundry groups of fishes and amphibians, the 
segmentation remains very imperfect: only certain peri- 
pheral appendages of the vertebre becoming defined and 
solidified, while in place of the bodies of the vertebra there 
still continues the undivided notochord. Thus, instead of 
being morphologically composed of vertebral segments, the 
vertebrate animal in its primitive form is entirely without 
vertebral segments; and vertebral segments begin to appear 
only as we advance towards developed forms. Once 
more, evidence equally adverse to the current hypothesis 
meets us on observing that the differences between the parts 
supposed to be homologous, are as great at first as at last. 
Did the vertebrate animal primordially consist of homo- 
logous segments from snout to tail; then the segments said 
to compose the skull ought, in the lowest Vertebrata, to show 
themselves much more like the remaining segments than 
they do in the highest Vertebrata. But they do not. Fishes 
have crania made up of bones that are no more clearly 
arrangeable into segments like vertebra, than are the cranial 
bones of the highest mammal. Nay, indeed, the case 1s 
much stronger: the simplest fish possessing a skeleton, 
has a cranium composed of cartilage that is not segmented 
at all! 

Besides being inconsistent with the leading truths of 
Embryology and Comparative Morphology, the hypothe- 
sis of Goethe and Oken is inconsistent with itself. The 
facts brought forward to show that there exists an arche- 


110 MORPHOLOGICAL DEVELOPMENT. 


typal vertebra ; and that the vertebrate animal is composed 
of archetypal vertebra arranged in a series, and sever- 
ally modified to fit their positions—these facts, I say, so far 
from proving as much, suffice, when impartially considered, 
to disprove it. No assigned nor any conceivable attribute of 
the supposed archetypal vertebra is uniformly maintained. 
The parts composing it are constant neither in their num- 
ber, nor in their relative positions, nor in their modes of 
ossification, nor in the separateness of their several individu- 
alities when present. There is no fixity of any one element, 
or connexion, or mode of development, which justifies even a 
suspicion that vertebree are modelled after an ideal pattern. 
To substantiate these assertions here would require too much 
space, and an amount of technical detail wearisome to the 
general reader. The warrant for them will be found in a 
eriticism on the osteological works of Prof. Owen, originally 
published in the British and Foreign Medico-Chirurgical 
Review for Oct. 1858. This criticism I add in the Appendix, 
for the convenience of those who may wish to study the 
question more fully. (See Appendix B.) 

Everything, then, goes to show that the segmental compo- 
sition which characterises the apparatus of external relation 
in most vertebrata, is not primordial or genetic, but function- 
ally determined or adaptive. Our inference must be that the 
vertebrate animal is an aggregate of the second order, in 
which a relatively superficial segmentation has been pro- 
duced by mechanical intercourse with the environment. We 
shall hereafter see that this conception leads us to a consist- 
ent interpretation of the facts—shows us why there has 
_ arisen such unity in variety as exists in every vertebral 
column, and why this unity in variety is displayed under 
countless modifications in different skeletons. 


§ 211. Glancing back at the facts brought together in 
these two chapters, it seems probable that there has gone on 
among animals a process parallel to that which we saw reason 


THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 111 


to think has gone on among plants. Minute aggregates of 
those physiological units which compose living protoplasm, 
exist as Protozoa: some of them incoherent, indefinite, and 
almost homogenous ; and others of them more coherent, de- 
finite, and heterogenous. By union of these nucleated parti- 
cles of sarcode, are produced various indefinite aggregates of 
the second order—Sponges, Thalassicolle, Foraminifers, &c. ; 
in which the compound individuality is scarcely enough 
marked to subordinate the primitive individualities. But in 
other types, as the Hydra, the lives of the morphological 
units are in a considerable degree, though not wholly, merged 
in the life of the integrated whole they form. As the primary 
ageregate when it passes a certain size undergoes fission or 
gemmation ; so does the secondary aggregate. And as on 
the lower stage so on the higher, we see cases in which the 
gemmiparously-produced individuals part as soon as formed, 
and other cases in which they continue united, though in great 
measure independent. This massing of secondary aggregates 
into tertiary aggregates, is variously carried on among the 
Hydrozoa, the Actinozoa, and the Molluscoida. In most of the 
‘types so produced, the component individualities are very 
little subordinated to the individuality of the mass they form 
—there is only physical unity and not physiological unity ; 
but in certain of the oceanic Hydrozoa, the individuals are so 
far differentiated and combined as very much to mask them. 
Forms showing us clearly the transition to well-developed 
individuals of the third order, are not to be found. Never- 
theless, in the great sub-kingdom Annulosa, there are traits 
of structure, development, and mode of multiplication, which 
go far to show that its members are such individuals of the 
third order; and in the relations to external conditions 
involved by the mode of union, we find an adequate cause for 
that obscuration of the secondary individualities which we 
must suppose has taken place. The two other great sub- 
kingdoms Mollusca and Vertebrata, between the lower mem- 
hers of which there are suggestive points of community, 


112 MORPHOLOGICAL DEVELOPMENT. 


present us only with aggregates of the second order, that 
have in many cases become very large and very complex. 
We find in them no trace of the union of gemmiparously- 
produced individuals. Neither the molluscous nor the 
vertebrate animal shows the faintest trace of a segmenta- 
tion affecting the totality of its structure; and we see 
good grounds for concluding that such segmentation as ex- 
ceptionally occurs in the one and usually occurs in the other, 
is superinduced. 


CHAPTER VI. 
MORPHOLOGICAL DIFFERENTIATION IN PLANTS. 


§ 212. Wutze in the course of their evolution plants and 
animals have displayed progressive integrations, there have 
at the same time been progressive differentiations of the 
resulting aggregates, both as wholes and in their parts. 
These differentiations and the interpretations of them, form 
the second class of morphological problems. 

We commence as before with plants. We have to con- 
sider, first, the several kinds of modification in shape they 
have undergone; and, second, the relations between these 
kinds of modification and their factors. Let us glance at 
the leading questions that have to be answered. 


§ 213. Irrespective of their degrees of composition, plants 
may, and do, become changed in their general forms. Are 
their changes capable of being formulated? The inquiry 
which meets us at the outset is—does a plant’s shape admit 
of being expressed in any universal terms ?—terms that 
remain the same for all genera, orders, and classes. 

After plants considered as wholes, have to be considered 
their proximate components, which vary with their degrees 
of composition, and in the highest plants are what we call 
branches. Is there any law traceable among the contrasted 
shapes of different branches in the same plant? Do the rela- 


tive developments of parts in the same branch conform to 
VOL, Il. 8 


114 MORPHOLOGICAL DEVELOPMENT. 


any law? And are these laws, if they exist, allied with one 
another and with that to which the shape of the whole 
plant conforms ? 

Descending to the components of these components, which 
in developed plants we distinguish as leaves, there meet us 
kindred questions respecting their relative sizes, their rela- 
tive shapes, and their shapes as compared with those of 
foliar organs in general, Of their morphological differentia- 
tions, also, it has to be asked whether they exemplify any 
truth that is exemplified by the entire plant and by its larger 
parts. 

Then, a step lower, we come down to those morphological 
units of which leaves and fronds consist; and concerning 
these arise parallel inquiries touching their divergencies 
from one another and from cells in general. 

The problems thus put together in several groups can- 
not of course be rigorously separated. Evolution pre-sup- 
poses transitions which make all such classings more or less 
conventional ; and adherence to them must be subordinate 
to the needs of the occasion. 


§ 214. In studying the causes of the morphological 
differentiations thus grouped and prospectively generalized, 
we shall have to bear in mind several orders of forces which 
it will be well briefly to specify. 

Growth tends inevitably to initiate changes in the 
shape of any aggregate, by changing both the amounts of 
the incident forces and the forces which the parts exert on 
one another. With the mechanical actions this is obvious: 
matter that is sensibly plastic cannot be increased in mass 
without undergoing a change in its proportions, consequent 
on the diminished ratio of its cohesive force to the force of 
gravitation. With the physiological actions it is equally 
obvious: increase of size, other things equal, alters the 
relations of the parts to the material and dynamical factors 
of nutrition ; and by so affecting differently the nutrition 


MORPHOLOGICAL DIFFERENTIATION IN PLANTS. 115 


of different parts, initiates further changes of propor- 
tions. 

Similarly in any composite plant, the proximate units as 
fast as they accumulate are subjected to mutual influences 
that are unlike one another and are continually changing. 
The earlier-formed units become mechanical supporters of the 
later-formed units, and so experience modifying forces from 
which the later-formed units are exempt. Further, these 
elder units simultaneously begin to serve as channels through 
which materials are carried to and from the younger units— 
another cause of differentiation that goes on increasing in in- 
tensity. Once more, there arise ever-strengthening contrasts 
between the amounts of light which fall upon the youngest or 
outermost units and the eldest or innermost units; whence 
result structural contrasts of yet another kind. Evidently, 
then, along with the progressive integration of cells into 
fronds, of fronds into axes, and of axes into plants still more 
composite, there come into play sundry causes of differen- 
tiation which act on the whole and on each of its parts, 
whatever their grade. The forces to be overcome, the forces 
to be utilized, and the matters to be appropriated, do not 
remain the same in their proportions and modes of action for 
any two members of the aggregate: be they members of the 
first, second, third, or any other order. 


_ § 215. Nor are these the only kinds and causes of hetero- 
geneity which we have to consider. Beyond the more 
general changes produced in the relative sizes and shapes of 
plants and their parts by progressive aggregation, there are 
the more particular changes determined by the more particu- 
lar conditions. 

Plants as wholes assume unlike attitudes towards their en- 
vironments; they have many ways of articulating their 
parts with one another; they have many ways of adjusting 
their parts towards surrounding agencies. These are causes of 
special differentiations additional to those general differentia- 

8 * 


116 MORPHOLOGICAL DEVELOPMENT. 


tions that result from increase of mass and increase of com- 
position. In each part considered individually, there arises 
a characteristic shape consequent on that relative position 
towards external and internal forces, which the mode of 
growth entails. very member of the aggregate presents 
itselfin a more or less peculiar way towards the light, towards 
the air, and towards its point of support ; and according to 
the relative homogeneity or heterogeneity in the incidence of 
the agencies thus brought to bear on it, will be the relative 
homogeneity or heterogeneity of its shape. 


§ 216. Before passing from this @ priori view of the mor- 
phological differentiations which necessarily accompany 
morphological integrations, to an a posteriori view of them, it 
seems needful to specify the meanings of certain descriptive 
terms we shall have to employ. 

Taking for our broadest division among forms, the regular 
and the irregular, we may divide the latter into those which 
are wholly irregular and those which, being but partially 
irregular, suggest some regular form to which they approach. 
By slightly straining the difference between them, two current 
words may be conveniently used to describe these subdivi- 
sions. The entirely irregular forms we may class as 
asymmetrical—liiterally as forms without any equality of 
dimensions. The forms which approximate towards regu- 
larity without reaching it, we may distinguish as wnsym- 
metricali—a word which, though it asserts inequality of 
dimensions, has been associated by use rather with such 
slight inequality as constitutes an observable departure from 
equality. 

Of the regular forms there are several classes, differing in 
the number of directions in which equality of dimensions is 
repeated. Hence results the need for names by which sym- 
metry of several kinds may be expressed. 

The most regular of figures is the sphere: its dimensions 
are the same from centre to surface in all directions; and if 


MORPHOLUGICAL DIFFERENTIATION IN PLANTS. TL? 


eut by any plane through the centre, the separated parts are 
equal and similar. This isa kind of symmetry which stands 
alone, and will be hereafter spoken of as spherical symmetry. 

When a sphere passes into a spheroid, either prolate or ob- 
late, there remains but one set of planes that will divide it 
into halves which are in all respects alike; namely, the 
planes in which its axis lies, or which have its axis for their 
line of intersection. Prolate and oblate spheroids may 
severally pass into various forms without losing this pro- 
perty. The prolate spheroid may become egg-shaped or py- 
riferm, and it will still continue capable of being divided into 
two equal and similar parts by any plane cutting it down 
its axis; nor will forming constrictions round it deprive it 
of this property. Similarly with the oblate spheroid. The 
transition from a slight oblateness like that of an orange 
to an oblateness reducing it nearly to a flat disc, does not 
alter its divisibility into like halves by every plane passing 
through its axis. And clearly the moulding of any such 
flattened oblate spheroid into the shape of a plate, leaves it 
as before, symmetrically divisible by all planes at right 
angles to its surface and passing through its centre. This 
species of symmetry is called radial symmetry. It is familiarly 
exemplified in such flowers as the daisy, the tulip, and the 
dahlia. | 

From spherical symmetry, in which we have an infinite 
number of axes through each of which may pass an infinite 
number of planes severally dividing the aggregate into equal 
and similar parts ; and from radial symmetry, in which we 
have a single axis through which may pass an infinite num- 
ber of planes severally dividing the aggregate into equal and 
similar parts ; we now turn to bilateral symmetry, in which 
the divisibility into equal and similar parts becomes very 
limited. Noting, for the sake of completeness, that there is 
a sextuple bilateralness in the cube and its derivative forms, 
which admit of division into equal and similar parts by planes 
passing through the three diagonal axes and by planes passing’ 


118 MORPHOLOGICAL DEVELOPMENT. 


through the three axes that join the centres of the surfaces, 
let us limit our attention to the three kinds of bilateralness 
which here concern us. The first of these 1s triple 
bilateral symmetry. ‘This is the symmetry of a figure having 
three axes at right angles to one another, through each of 
which there passes a single plane that divides the aggregate 
into corresponding halves. A common brick will serve as an 
example; and of objects not quite so simple, the most familiar 
is that modern kind of spectacle-case which is open at both 
ends. This may be divided into corresponding halves along 
its longitudinal axis, by cutting it through in the direction 
of its thickness or by cutting it through in the direction of 
its breadth ; or it may be divided into corresponding halves 
by cutting it across the middle. Of objects which 
illustrate double bilateral symmetry, may be named one of 
those boats built for moving with equal facility in either di- 
rection, and therefore made alike at stem and stern. Ob- 
viously such a boat is separable into equal and similar parts - 
by a vertical plane passing through stem and stern ; and it is 
also separable into equal and similar parts by a vertical plane 
cutting it amidships. To exemplify single bilateral 
symmetry it needs but to turn to the ordinary boat of which 
the two ends are unlike. Here there remains but the one 
plane passing vertically through stem and stern, on the op- 
posite sides of which the parts are symmetrically disposed. 
These several kinds of symmetry as placed in the fore- 
going order, imply increasing heterogeneity. The greatest 
uniformity in shape is shown by the divisibility into like 
parts in an infinite number of infinite series of ways ; and 
the greatest degree of multiformity consistent with any 
regularity, is shown by the divisibility into like parts in 
only a single way. Hence, in tracing up organic evolution 
as displayed in morphological differentiations, we may ex- 
pect to pass from the one extreme of spherical symmetry, 
to the other extreme of single bilateral symmetry. This 
expectation we shall find to be completely fulfilled. 


CHAPTER VII. 


THE GENERAL SHAPES OF PLANTS. 


§ 217. Amone protophytes those which are by general 
consent regarded as the simplest, are the Protococci. As 
shown in Fig. 1, they are globular cells presenting no ob- 
vious differentiation save that between inner and outer parts. 
Their uniformity of figure coexists with a mode of life involv- 
ing the uniform exposure of all their sides to incident forces. 
The Protococcus nivalis, which colours red the snow through 
which it spreads with such marvellous rapidity, is subject to 
no constant contrasts in the amounts of light, heat, air, or 
moisture, on its upper and lower surfaces. For though each 
individual may have its external parts differently related to 
environing agencies, yet the new individuals produced by 
spontaneous fission have no means of maintaining parallel 
relations of position among their parts. On the contrary, 
the indefiniteness of the attitudes into which successive 
generations fall, must prevent the rise of any unlikeness be- 
tween one portion of the surface and another. Spherical 
symmetry continues because, on the average of cases, inci- 
dent forces are equal in all directions. 

Other orders of Protophyta have much more special 
forms, along with much more special attitudes: their ho- 
mologous parts maintaining, from generation to generation, 
unlike relations to incident forces. The Desmidiacee and 


120 MORPHOLOGICAL DEVELOPMENT. 


Diatomacee, of which Figs. 2 and 3 show examples, severally 
include genera characterized 
by triple bilateral sym- 
‘| metry. A Navicula is di- 
visible into corresponding 
halves by a transverse plane 
and by two longitudinal planes—one cutting its valves at 
right angles and the other passing between its valves. The 
like is true of those numerous transversely-constricted forms 
of Desmidiacee, exemplified by the second of the individuals 
represented in Fig. 2. If now we ask how a Wavicula is re- 
lated to its environment, we see that its mode of life exposes 
it to three different sets of forces: each set being resolvable 
into two equal and opposite sets. A Navicula moves in the 
direction of its length, with either end foremost. Hence, on 
the average, its ends are subject to like actions from the 
agencies to which its motions subject it. Further, either 
end while moving, exposes its right and left sides to amounts 
of influence which in the long run must be equal. If, then, 
the two ends are not only like one another, but have cor- 
responding right and left sides, the symmetrical distribu- 
tion of parts answers to the symmetrical distribution of 
forces. Passing to the two edges and the two flat sur- 
faces, we similarly find a clue to their likenesses and differ- 
ences in their respective relations to the things around them. 
These locomotive protophytes move through the entangled 
masses of fragments and fibres produced by decaying organ- 
isms and confervoid growths. The interstices in such matted 
accumulations are nearly all of them much longer in one 
dimension than in the rest—form crevices rather than 
regular meshes. Hence, a small organism will have much 
greater facility of insinuating itself through this débris, in 
which it finds nutriment, if its transverse section is flattened 
instead of square: or circular. And while we see how, by 
survival of the fittest, a flattened form is likely to be ac- 
quired by diatoms having this habit ; we also see that like- 


THE GENERA. SHAPES OF PLANTS. 121 


ness Will be maintained between the two flat surfaces and 
between the two edges. For, on the average, the relations 
of the two flat surfaces to the sides of the openings through 
which the diatom passes, will be alike; and so, too, on the 
average, will be the relations of the two edges. In 
desmids of the type exemplified by the second individual in 
Fig. 2, a kindred equalization of dimensions is otherwise in- 
sured. There is nothing to keep one of the two surfaces 
uppermost rather than the other; and hence, in the long 
succession of individuals, the two surfaces are sure to be 
similarly exposed to light and agencies in general. When 
to this is added the fact that spontaneous fission occurs 
transversely in a constant way, it becomes manifest that the 
two ends, while they are maintained in conditions lke one 
another, are maintained in conditions unlike those of the two 
edges. Here then, as before, triple bilateral symmetry in 
form, coexists with a triple bilateral symmetry in the 
average distribution of actions. 

Still confining our attention to aggregates of the first 
order, let us next note what results when the two ends are 
permanently subject to different conditions. The fixed 
unicellular plants, of which examples are given in Figs. 4, 5, 
and 6, severally illustrate the contrast in shape that arises 


and the part that extends into the surrounding medium. 
These two parts which are the most unlike in their relations 
to incident forces, are the most unlike in their jorms. Ob- 


122 MORPHOLOGICAL DEVELOPMENT. 


serve, next, that the part which lifts itself into the water or 
air, is more or less decidedly radial. Each upward growing 
tubule of Codiwm adherens, Fig. 4, has its parts disposed 
with some regularity around its axis; the upper stem and 
spore-vessel of Hydrogastrum, Fig. 5, display a lateral 
growth that is approximately equal in every direction; and 
the branches of the Botrytis, Fig. 6, shoot out with an ap- 
proach to evenness on all sides. Plants of this low type 
are naturally very variable in their modes of growth: each 
individual being greatly modified in form by its special cir- 
cumstances. But they nevertheless show us a general like- 
ness between parts exposed to like forces, as well as a general 
unlikeness between parts exposed to unlike forces. 
Respecting the forms of these aggregates of the first order, 
it has only to be added that they are asymmetrical where 
there is total irregularity in the incidence of forces. We 
have an example in the indefinitely contorted and branched 
shape of a fungus-cell, growing as a mycelium among the 
particles of soil or through the interstices of organic tissue. 


§ 218. Re-illustrations of the general truths which the 
forms of these vegetal aggregates of the first order display, 
are furnished by vegetal aggregates of the second order. 
The equalities and inequalities of growth in different direc- 
tions, prove to be similarly related to the equalities and in- 
equalities of environing actions in different directions. 

Of spherical symmetry, an instance occurs in the Volvox 
globator. The ciliated cells, here so united as to produce a 
small, mulberry-shaped, hollow ball, cause, by the movements 
of their cilia, a simultaneous rotation of the ball and pro- 
gress of it through the water. There is nothing to de- 
termine the axis of rotation or the direction of rotation.’ 
And if the axis and direction of rotation continually vary, 
as we may conclude that they do, then the different mem- 
bers of the aggregate severally occupy in their turns like 
positions towards surrounding agencies; and so are not 


att 


THE GENERAL SHAPES*OF PLANTS. 128 


made to lose their homogeneity of form and distribution. 

Vegetal aggregates of the second order are usually fixed : 
locomotion is exceptional. Fixity implies that the surface 
of attachment is differently circumstanced from the free sur- 
face. Hence we may expect to find, as we do find, that 
among these rooted aggregates of the second order, as among 
those of the first order, the primary contrast of shape is 
between the adherent part and the loose part. Sea-weeds 
variously exemplify this. In some the fronds are very 
irregular and in some tolerably regular; in some the form is 
pseudo-foliar and in some pseud-axial; but differing though 
they do in these respects, they agree in having the end 
which is attached to a solid body unlike the other end. The 
same truth is seen in such secondary aggregates as the com- 
mon fungi, or rather in their immensely-developed organs of 
fructification. A puff-ball, Fig. 192, presents no other 
obvious unlikeness of parts than that between its under and 
upper surfaces. So too with the stalked kinds that frequent 
our woods and pastures. In the types which Figs. 198, 
194, 195, delineate, the unlikenesses between the rooted 
ends and the expanded ends, as well as between the under 
and upper surfaces of the expanded ends, are obviously 
related to this fundamental contrast of conditions. Nor is 
this relation less clearly displayed in the sessile fungi which 
grow out from the sides of trees, as shown at a, ), Fig. 
196. That which is common to this and the preceding types, 
is the contrast between the attached end and the free 
end. 

From what these forms have in common, let us turn 
to that which they have not in common, and observe the 
causes of the want of community. A. puff-ball shows us 
in the simplest way, the likeness of parts accompanying 
likeness of conditions, along with the unlikeness of parts 
accompanying unlikeness of conditions. For while, if we 
cut vertically through its centre, we find a difference be- 
tween top and bottom, if we cut horizontally through its 


124 MORPHOLOGICAL DEVELOPMENT. 


centro, we find no differences among its several sides. 
Being, on the average of cases, similarly related to the envi- 
ronment all round, it remains the same all round. The 
radial symmetry of the mushroom and other vertically- 


growing fungi, illustrates this connexion of cause and 
effect still better. But now mark what happens in the 
group of Agaricus xylophilus, shown in Fig. 195. Radi- 
ally symmetrical as is the type, and radially symmetri- 
cal as are those centrally-placed individuals which are 
equally crowded all round, we see that the peripheral indi- 
viduals, dissimilarly cireumstanced on their outer sides and 
on their sides next the group, have partially changed their 
radial symmetry into bilateral symmetry. It is no longer 
possible to make two corresponding halves by any vertical 
plane cutting down through the pileus and the stem; but 
there is only one vertical plane that will thus produce cor- 
responding halves—the plane on the opposite sides of which 
the relations to the environment are alike. And then mark 
that the divergence from all-sided symmetry towards two- 
sided symmetry, here caused in the individual by special 
circumstances, is characteristic of the race where the 
habits of the race constantly involve two-sidedness of condi- 
tions. Besides being exemplified by such comparatively 
undifferentiated types as Boletus, Fig. 196, a, b, this truth 
is exemplified by members of the genus just named. In 
Agaricus horizontalis, Fig. 196, c, we kave a departure from 
radial symmetry that is conspicuous only in the form of the 
stem. A more decided bilateralness exists in A. palmatus, 


THE GENERAL SHAPES OF PLANTS. 125 


shown in elevation at d and in section at d’. And A. flabelli- 
formis, of which e and é are different views, exhibits com- 
plete bilateralness—a bilateralness in which there is the 
greatest likeness of the parts that are most similarly condi- 
tioned, and the greatest unlikeness of the parts that are most 
dissimilarly conditioned. 

Among plants of the second order of composition, it will 
suffice to note one further class of facts which are the con- 
verse of the foregoing and have the same implications. These 
are the facts showing that along with habitual irregularity in 
the relations to external forces, there is habitual irregularity 
in the mode of growth. Besides finding such facts among 
Thallogens, as in the tubers of underground fungi and in the 
creeping films of sessile lichens, which severally show us 
variations of proportions obviously caused by variations in the 
amounts of the influences on their different sides, we also 
among Acrogens of inferior types, find irregularities of form 
along with irregularities in environing actions. The fronds 
of the Marchantiacee or such Jungermanniacee as are shown in 
Figs. 41, 42, 43, illustrate the way in which each lowly-or- 
ganized aggregate of the second order, not individuated by 
the mutual dependence of its parts, has its form determined 
by the balance of facilities and resistances which each portion 
of the frond meets with as it spreads. 


§ 219. Among plants that display integration of the third 
degree, and among plants still further compounded, these 
same truths are equally manifest. In the forms of such 
plants we see primary contrasts and secondary contrasts, 
which, no less clearly than the foregoing, are related to 
contrasts of conditions. 

That flowering plants from the daisy up to the oak, have 
in common the fundamental unlikeness between the upward 
growing part and the downward growing part; and that 
this most marked unlikeness corresponds with the most mark- 
ed unlikeness between the two parts of their environment, 


126 MORPHOLOGICAL DEVELOPMENT. 


soil and air; are facts too conspicuous to be named were they 
not important items in the argument. More instructive, 
perhaps, because less familiar, is the fact that we miss this 
extreme contrast in flowering plants which have not their 
higher and lower portions exposed to conditions so extremely 
contrasted. A parasite like the Dodder, growing in entan- — 
gled masses upon other plants, from which it sucks the juices, 
is not thus divisible into two strongly-distinguished halves. 

Leaving out of consideration the difference between the 
supporting part and the supported part in phenogams, and 
looking at the supported part only, we observe between its 
form and the habitual incidence of forces, a relation like that 
which we observed in the simpler plants. Phenogams that 
are practically if not literally uniaxial, and those which de- 
velop their lateral axes only in the shape of axillary flowers, 
when uninterfered with ordinarily send up vertical axes round 
which the leaves and flowers are disposed with a more or less 
decided radial symmetry. Gardens and fields supply us 
with such instances as the Tulip and the Orchis; and ona 
larger scale the Palms and the Aloes are fertile in examples. 
The exceptions, too, are instructive. Besides the individual 
divergences that arise from special interferences, there are 
to be traced general divergences where the habits of the 
plants expose them to general interferences in anything 
approaching to constant ways. Plants which, like the Fox- 
glove, have spikes of flowers that are borne on flexible foot- 
stalks, have their flowers habitually bent round to one face of 
the stem: an unlikeness of distribution probably caused by 
unlikeness in the relation to the sun’s rays. The wild Hya- 
cinth, too, with stem so flexible that its upper part droops, 
shows us how a consequent difference in the action of gravity 
on the flowers, causes them to deviate from their typically 
radial arrangement towards a bilateral arrangement. 

Much more conspicuous are the segeneral and special re- 
lations of form to general and special actions in the environ- 
ment, among phenogams that are multiaxial. That when 


THE GENERAL SHAPES OF PLANTS. 127 


standing alone, and in positions where the winds do not injure 
them or adjacent objects shade them, shrubs and trees develop 
with tolerable evenness on all sides, is an obvioustruth. Equal- 
ly obvious is the truth that, when growing together in a wood, 
and mutually interfered with on all sides, trees still show 
obscurely radial distributions of parts; though, under such 
conditions, they have tall taper stems with branches directed 
upwards—a difference of shape clearly due to the different 
incidence of forces. And almost equally obvious is the truth, 
that a tree of this same kind growing at the edge of the wood, 
has its outer branches well-developed and its inner branches 
comparatively ill-developed. Fig. 197, which very inaccur- 


i | : y 


wae eri) 299 


ately represents this difference, will serve to make it manifest 
that while one of the peripheral trees can be cut into some- 
thing like two similar halves by a vertical plane directed to- 
wards the centre of the wood—a plane on each side of which 
the conditions are alike—it cannot be cut into similar halves 
by any other plane. A like divergence from an indefinitely- 
radial symmetry towards an indefinitely-bilateral symmetry, 
occurs in trees that have their conditions made bilateral by 
growing on inclined surfaces. Two of the common forms 
observable in such cases are given in Fig. 198. Here there 
is divisibility into parts that are tolerably similar, hy a vertical 
plane running directly down the hill; but not by any other 
plane. Then, further, there is the bilateralness, similar in 
general meaning though differently caused, which we see 
in trees exposed to strong prevailing winds. Almost every 


128 MORPHOLOGICAL DEVELOPMENT. 


sea-coast has abundant examples of stunted trees which, like 
the one shown in Fig. 199, have been made to deviate from 
their ordinary equal growth on all sides of a vertical axis, to 
a growth that is equal only on the opposite sides of a vertical 
plane directed towards the wind’s eye. 

From among vegetal aggregates of the third order, we have 
now only to add examples of the entirely asymmetrical form 
that accompanies an entirely irregular distribution of inci- 
dent forces. Creeping plants furnish such examples. They 
show us, alike when climbing up vertical or inclined surfaces 
or trailing along the ground, that their branches grow hither 
and thither as the balance of forces aids or opposes ; and the 
general outline is without symmetry of any kind, because 
the environing influences have no kind of regularity in their 
arrangement. 


§ 220. Along with some unfamiliar facts, I have here set 
down facts that are so familiar as to seem scarcely worth 
noting. It is because these facts have become meaningless 
to perceptions deadened by infinite repetitions of them, that 
it is needful here to point out their meaning. Not alone for 
its intrinsic importance has the unlikeness between the 
attached ends and the free ends been traced among plants 
of all degrees of integration. Nor is it simply because of the 
significance they have in themselves, that instances have been 
given of those different varieties of symmetry and asymmetry 
which the free ends of plants equally display : be they plants 
of the first, second, third, or any higher order. Neither has 
the only other purpose been that of showing how, in the radial 
symmetry of some vegetal aggregates and the single bilateral 
symmetry of others, there are traceable the same ultimate 
principles as in the spherical symmetry and triple bilateral 
symmetry of certain minute plants first described. But the 
main object has been to present under their simplest aspects, 
those general laws of morphological differentiation which are 
fulfilled by the component parts of each plant. 


THE GENERAL SHAPES OF PLANTS. 129 


If organic form is determined by the distribution of forces, 
and the approach in every case towards an equilibrium of 
inner actions with outer actions; then this relation between 
forms and forces must hold alike in the organism asa whole, 
in its proximate units, and in its units of lower orders. Formu- 
las which express the shapes of entire plants in terms of sur- 
rounding conditions, must be formulas which also express 
the shapes of their several parts in terms of surrounding 
conditions. If, therefore, we find that a plant as a whole is 
radially symmetrical or bilaterally symmetrical or asymme- 
trical, according as the incident forces affect it equally on all 
sides of an axis or affect it equally only on the opposite sides 
of one plane or affect it equally in no two directions; then, 
we may expect that each member of a plant will display radial 
symmetry where environing influences are alike along many 
radii, bilateral symmetry where there is bilateralness of 
environing influences, and unsymmetry or asymmetry where 
there is partial or entire departure from a balance of sur- 
rounding actions. 

To show that this expectation is borne out by the facts, 
will be the object of the following four chapters. Let us 
begin with the largest parts into which plants are divisible ; 
and proceed to the successively smaller parts. 


CHAPTER VIII. 


THE SHAPES OF BRANCHES. 


§ 221. Accrrcarss of the first order supply a few examples 
of forms ramified in an approximately-regular manner, under 
conditions which subject their parts to approximately-regu- 
lar distributions of forces. Some unicellular Alg@, becoming 
elaborately branched, assume very much the aspects of small 
trees ; and show us in their branches analogous relations of 
forms to forces. Bryopsis plumosa may 
be instanced. Fig. 200 represents the 
end of one of its lateral ramifications, 
above and beneath which come others of 
like characters. Here it will be seen that 
the attached and free ends differ; that 
the two sides are much alike; and that they are unlike the 
upper and under surfaces, which resemble one another. 


§ 222. Hig. 201 shows us how in an aggregate of the se- 
cond order, each proximate component 1s 
modified by its relations to the rest ; just 
as we before saw a whole fungus of the 
same type modified by its relations to en- 
vironing objects. Ifa branch of the fun- 
gus here figured, be compared with one of 
the fungi clustered together in Fig. 195, 
or, still better, with one of the laterally- 


THE SHAPES OF BRANCHES. 13l 


growing fungi shown in Fig. 196, there will be perceived a 
kindred transition from radial to bilateral symmetry, occurring 
under kindred conditions. The portion of the pileus next to 
' the side of attachment is undeveloped in this branched form 
as in the simple form ; and in the one case as in the other, 
the stem is modified towards the side of attachment. A di- 
vision into similar halves, which, as shown in Fig. 196 é, might 
be made of the whole fungus by a vertical plane passing 
through the centre of the pileus and the axis of the support- 
ing body, might here be made of the branch, by a vertical 
plane passing through the centre of its pileus and the axis of 
the main stem. Among aggregates of this order, the Alge 
furnish cases of kindred nature. In the branches of Lessonia, 
Fig. 37, may be observed a substantially-similar relationship : 
their inner parts being less developed than their outer parts, 
while their two sides are developed in approximately equal 
degrees, they are rendered bilateral. 


§ 223. These few cases introduce us to the more familiar 
but more complex cases which plants of the third degree of 
aggregation present. Ata, b,c, Fig. 202, are sketched three 


homologous parts of the same tree: a being the leading 

shoot; } a lateral branch near the top, and c a lateral 

branch lower down. There is here a double exemplifica- 

tion. While the branch o, as a whole, has its branchlets 
o7e , 


132 MORPHOLOGICAL DEVELOPMENT. 


arranged with tolerable regularity all round, in corre- 
spondence with its equal exposure on all sides, each branch- 
let shows by its curve as much bilateral symmetry as 
its simple form permits. The branch 6, dissimilarly 
circumstanced on the side next the main stem and on 
the side away from it, has an approximate bilateralness 
as a whole, while the bilateralness of its branchlets varies 
with their respective positions. And m the branch ¢, having 
its parts still more differently conditioned, these traits of 
structure are still more marked. Ixtremely strong contrasts 
of this kind occur in trees having very regular modes of 
growth. The uppermost branches of a Spruce-fir have radially 
arranged branchlets: each of them, if growing vigorously, 
repeats the type of the leading shoot, as shown in Fig. 203, 
a, b. But if we examine branches lower and lower down the 
tree, we find the vertically-growing branchlets bear a less and 
less ratio to the horizontally-growing ones; until, towards the 
bottom, the radial arrangement has wholly merged into the 
bilateral. Shaded and confined by the branches above them, 
these eldest branches develop their offshoots in those direc- 
tions where there is most space and light: becoming’ finally 
quite flattened and fan-shaped, as shown at Fig. 203, ¢c. And 
on remembering that each of these eldest branches, when first 
it diverged from the main stem, was radial, we see not only 
that between the upper and lower branches does this contrast 
in structure hold, but also that each branch is transformed 
from the radial to the bilateral by the progressive change in 
its environment. Other forces besides those which aid 
or hinder growth, conspire to produce this two-sided character 
in lateral branches. The annexed Fig. 204, sketched from 
an example of the Pinus Ooulterii at Kew, shows very clearly 
how, by mere gravitation, the once radially-arranged. branch- 
lets may be so bent as to produce in the branch asa whole a 
decided bilateralness. A full-grown Araucaria, too, exhibits 
in its lower branches modifications similarly caused; and in 
each of such branches there may be remarked the further fact, — 


THE SHAPES OF BRANCHES. 133 


that its upward-bending termination hasa partially-modified 
radialness, at the same time that its drooping lateral branch- 
lets give to the part nearer the trunk a completely bilateral 
character. 

Now in these few instances, which are typical of countless 
instances that might be given, we see, a8 we saw in the case 
of the fungi, that the same thing is true of the parts in 
their relations to the whole and to one another, which is true 
of the whole in its relations to the environment at large. 
Entire trees become bilateral instead of radial, when exposed 
to forces that are equal only on opposite sides of one plane ; 
and in their branches, parallel changes of form occur under 
parallel changes of conditions. 


§ 224. There remains to be said something respecting the 
distribution of leaves. How a branch carries its leaves con- 
stitutes one of its characters as a branch; and is to be con- 
sidered apart from the characters of the leaves themselves. 
The principles hitherto illustrated we shall here find illus- 
trated still further. 

The leading shoot and all the upper twigs of a fir-tree, 
have their pin-shaped leaves evenly distributed all round, or 
placed radially ;* but as we descend, we find them beginning 
to assume a bilateral distribution; and on the lower, hori- 
zontally-growing branches, their distribution is quite bilateral. 
Between the Irish and English kinds of Yew, there is a con- 
trast of like significance. The branches of the one, shooting 
up as they do almost vertically, are clothed with leaves 
all round ; while those of the other, which spread laterally, 
bear their leaves on the two sides. In trees with better- 
developed leaves, the same principle is more or less manifest 
in proportion as the leaves are more or less enabled by their 
structures to maintain fixed positions. Where the foot-stalks 


~* Here and throughout, the word radial is applied equally to the spiral and 
the whorled structures. These, as being alike on all sides, are similarly distin- 
guished from arrangements that are alike on two sides only. 


134 MORPHOLOGICAL DEVELOPMENT. 


are long and slender, and where, consequently, each leaf, ac- 
cording to its weight, the flexibility and twist of its foot- 
stalk, and the direction of the branch it grows from, falls 
into some indefinite attitude, the relations are obscured. But 
where the foot-stalks are stiff, as in the Laurel, it will be 
found, as before, that from the topmost and upward-growing 
branches the leaves diverge on all sides; while the under- 
most branches, growing out from the shade of those above, 
have their leaves so turned as to bring them into rows hori- 
zontally spread out on the two sides of each branch. 
A kindred truth, having like implications, comes into view 
when we observe the relative sizes of leaves on the same 
205 branch, where their sizes differ. 
Co Fig. 205 represents a branch of a 
Horse-chesnut, taken from the low- 


. a 
i ermost fringe of the tree, where the 


4 light has been to a great extent in- 

| tercepted from all but the most pro- 

fs WS truded parts. Beyond the fact that 

the leaves are bilaterally distributed 

b on this drooping branch, instead of 

being distributed symmetrically all round, as on one of the 

ascending shoots, we have here to note the fact that there 

is unequal development on the upper and lower sides. Each 

of the compound leaves acquires a foot-stalk and leaflets that 

are large in proportion to the supply of light ; and hence, as 

we descend towards the bottom of the tree, the clusters of 

leaves display increasing contrasts. How marked these con- 

trasts become will be seen on comparing a and b, which form 

one pair of leaves that are normally equal, or c and d, which 
form another pair normally equal. 

Let us not omit to note, while we have this case before us, 
the proof it affords that these differences of development are 
in a considerable degree determined by the different con- 
ditions of the parts after they have been unfolded. Though 
those inequalities of dimensions whence the differentiations 


THE SHAPES OF BRANCHES. 135 


of form result, are in many cases largely due to the inequali- 
ties in the circumstances of the parts while in the bud (which 
are however representative of inequalities in ancestral cir- 
cumstances) ; yet these are clearly not the sole causes of the 
unlikenesses that eventually arise. For the leaf-buds whence 
the larger leaves in Fig. 205 were developed, instead of being 
at first more favourably circumstanced than the others, were 
less favourably circumstanced. So that this bilateralness 
that results from the unequal sizes of the leaves, must be con- 
sidered as wholly due to the differential actions that come into 
play after the leaves have assumed their typical structures. 


§ 225. How in the arrangement of their twigs and leaves, 
branches tend to lapse from forms that are approximately 
symmetrical to forms that are quite asymmetrical, need not 
be demonstrated: it is sufficiently conspicuous. But it may 
be well to point out how the tendency to do this further 
enforces our argument. The comparatively regular budding- 
out of secondary axes and tertiary axes, does not usually 
produce an aggregate which maintains its regularity, for 
the simple reason that many of the axes abort. Terminal 
buds are some of them destroyed by birds; others are bur- 
rowed into by insects; others are nipped by frost; others 
are broken off or injured during gales of wind. The envi- 
ronment of each branch and its branchlets is thus ever 
being varied on all sides: here, space being left vacant by 
the death of some shoot that would ordinarily have occupied 
it; and there, space being trenched on by the lateral growth 
of some adjacent branch that has had its main axis broken. 
Hence the asymmetry or heterogeneity of form which the 
branch assumes, is.caused by the asymmetrical distribution 
of incident forces—a result and a cause that go on ever com- 


plicating. 


§ 226. One conspicuous trait in the shapes of branches 
has still to be named. Their proximal or attached ends 
differ from their distal or free ends, in the same way that 


136 MORPHOLOGICAL DEVELOPMENT. 


the lower ends of trees differ from their upper ends. This 
fact, like the fact to which it is here paralleled, has had its 
significance obscured by its extreme familiarity. But it 
shows in a striking way how the most differently-conditioned 
parts become the most strongly contrasted in their struc- 
tures. A phnogamic axis is made up of homologous 
segments, marked off from one another by the nodes; and 
a compound branch consists of groups of such segments. The 
earliest-formed segments, alike of the tree and of each 
branch, serve as mechanical supports and channels for sap 
to the successive generations of segments that grow out of 
them; and become more and more shaded by their pro- 
geny as these increase. Hence the progressively-increasing 
contrasts. If the trunk of a tree were sawn horizontally 
into a series of slabs, each some two inches thick or there- 
abouts; if each of the main branches were similarly divided 
transversely, and the like were done with all the branches 
borne by it, down to their ultimate twigs, which would be se- 
verally cut across at each internode; then, morphologically 
considered, any one of these slabs would be the homologue 
of any internode of an ultimate twig, with its leaf and axil- 
lary bud. In the immense contrast between these oldest 
and youngest units of composition, we should have exhibited 
the cumulative result of continuous differentiation caused by 
continuous action of modifying forces—the one unit having 
been originally just like the other. 


§ 227. Thus, then, it is with the proximate parts of plants as 
it is with plants as wholes. The radial symmetry, the bilateral 
symmetry, and the asymmetry, which branches display in 
different trees, in different parts of the same tree, and at 
different stages of their own growths, prove to be all conse- 
quent on the ways in which they stand towards the entire 
plexus of surrounding actions. The principle that the 
growths are unequal in proportion as the relations to the 
environment are unequal, serves to explain all the leading 
traits of structure. 


CHAPTER IX. 
THE SHAPES OF LEAYES. 


§ 228. Nexr in the descending order of composition come 
compound leaves. ‘lhe relative sizes and distributions of 
their leaflets, as affecting their forms as wholes, have to be 
considered in their relations to conditions. Figs. 206, 207, 
represent leaves of the common Ovalis and of, the Marsilea, 
in which radial symmetry is as completely displayed as the 
small number of leaflets permits. This equal development 
of the leaflets on all sides, occurs where the foot-stalks, grow- 
ing up vertically from creeping or underground stems, are 
so long that the leaves either do not interfere with one 
another or do it in an inconstant way: the leaflets are not 
differently conditioned on different sides, as they are where 
the foot-stalks grow out in the ordinary manner. How un- 
likeness of position influences the leaflets is clearly shown in 
a Clover-leaf, Fig. 208, which deviates from the Oxalis-leaf 
but slightly towards bilateralness, as it deviates from it but 
slightly in the attitude of its petiole; which is a little in- 
clined away from the others borne by the same procumbent 
axis. A familiar example of an almost-radial symmetry 
along with almost equal relations to surrounding conditions, 
occurs in the root-leaves of the Lupin, Fig. 209, 6. Here 
though we have lateral divergence from a vertical axis, yet 
the long foot-stalks preserve nearly erect positions, and 
carry their leaves to such distances from the axis, that the 


138 MORPHOLOGICAL DEVELOPMENT. 


development of the leaflets on the side next the axis is not 
much hindered. Still the interference of the leaves with one 
another is, on the average, somewhat greater on the proximal 
side than on the distal side; and hence the interior leaflets 
are rather less than the exterior leaflets. In further proof of 
which influence, let it be added that, as shown in the figure, 
at a, the leaves growing out of the flowering-stem devi- 
ate towards the two-sided form more decidedly. Two- 
sidedness is much greater where there is a greater relative 
proximity of the inner leaflets to the axis, or where the foot- 
stalk approaches towards a horizontal position. The Horse- 
chesnut, Fig. 205, already instanced as showing how the 
arrangements and sizes of leaves are determined by the 
incidence of forces, serves also to show how the incidence 
of forces determines the relative sizes and arrangements 


of leaflets. Fig. 210, which shows a leaf of the 


Bombaz, further illustrates this relation of structure to con- 
ditions. 

Compound leaves that are completely bilateral, present us 
with modifications of form exemplifying the same general 
truth in another way. In them the proximal and distal 
parts have none of that resemblance which we see in those 
intermediate forms just described: the portion next the axis 
and the portion furthest from the axis are entirely different ; 
and the only likeness is between the wings or leaflets on 


THE SHAPES OF LEAVES. 139 


opposite sides of the main foot-stalk or midrib. On turning 
back to Fig. 65, it will be seen that the compound leaf there 
drawn to exemplify another truth, serves also to exemplify this 
truth: the homologous parts a, b, c, d, while they are unlike 
one another, are, in their main proportions, severally like 
the parts with which they are paired. And here let us not 
overlook a characteristic which is less conspicuous but not 
less significant. Each of the lateral wings has winglets 
that are larger on the one side than on the other; and in 
each case the two sides are dissimilarly conditioned. Even 
in the several components of each wing may be traced a like 
divergence from symmetry, along with a like inequality in 
the relations to the rest: the proximal half of each leaflet 
is habitually larger than the distal half. In the leaves of 
the Bramble, previously figured, kindred facts are presented. 
How far such differences of development are due to the posi- 
tions of the parts in the bud; how far the respective 
spaces available for the parts when unfolded affect them; 
and how far the parts are rendered unlike by unlikenesses in 
their relations to light; it is difficult to say. Probably 
these several factors operate in all varieties of proportion. 
That the habitual shading of some parts by others largely 
aids in causing these divergences from symmetry, is very 
instructively shown by the compound leaves of the Cow- 
parsnip. Fig. 211 represents one of these. While the leaf as a 


whole is bilaterally symmetrical, each of the wings has an un- 
symmetrical bilateralness : the side next the axis being larger 
than the remoter side. How does this happen? Fig. 212, 


140 MORPHOLOGICAL DEVELOPMENT 


which is a diagrammatic section down the midrib of the 
leaf, showing its inclined attitude and the positions of the 
wings a, b, c, will make the cause clear. As the wings 
overlap like the bars of a Venetian blind, each intercepts 
some light from the one below it; and the one below it 
thus suffers more on its distal side than on its proximal side. 
Hence the smaller development of the distal side. That this 
is the cause is further shown by the proportion that is main- 
tained between the degree of obscuration and the degree of 
non-development ; for this unlikeness is greater between the 
two sides a and a’, than between 0 and 0’, or c and ¢, at the 
same time that the interference is greater in the lower wings 
than in the upper. Of course in this case and in the kindred 
cases hereafter similarly interpreted, it is not meant that this 
differentiation is consequent solely, or even chiefly, on 
the differential actions experienced by the individual plant: 
Though there is good reason to believe that the rate of growth 
in each part of each leaf is affected by the incidence of light, 
yet contrasts so marked and so systematic as these are not 
explicable without taking into account the inheritance of 
modifications either functionally caused or caused by spon- 
taneous variation. Clearly, the tendency will be towards 
the preservation of a plant which distributes its chlorophyll 
in the most economical way ; and hence there will always be 
a gravitation towards a form in which shaded parts of leaves 
are undeveloped. 


§ 229. From eompound leaves to simple ones, we find 
transitions in leaves of which the divisions are partial in- 
stead of total; and in thesé we see, with equal clearness, the 
relations between forms and positions that have been traced 
thus far. Fig. 213 is the leaf of a Winter-aconite, in which, 
round a vertical petiole, there is a radial distribution of half- 
separated leaflets. The Cecropia-leaf, Fig. 214, shows us a 
two-sided development of the parts beginning to modify, 
but not obliterating, the all-sided arrangement; and this 


THE SHAPES OF LEAVES. V1 


mixed symmetry occurs under conditions that are interme- 
diate. A more marked degree of the same relation is pre- 
sented in the leaf of the Lady’s Mantle, Fig. 215. And 


BNZ 


| be mx 
~~” 


then in the Sycamore and the Vine, we have a cleft type of 
leaf in which a decided bilateralness of form co-exists with 
a decided bilateralness of conditions. 

The quite simple leaves to which we now descend, exhibit, 
very distinctly, a parallel series of facts. Where they grow 
up on long and completely-independent foot-stalks, without 

definite subordination to some central vertical axis, the 
leaves of water-plants are symmetrically peltate. Of this 
the sacred Indian-bean, Fig. 216, furnishes an example. Here 
there is only a trace of bilateralness in the venation of the 
leaf, corresponding to the very small difference of the con- 
ditions on the proximal and distal sides. In the Vectoria 
regia, Fig. 217, the foot-stalks, though radiating almost 
horizontally from a centre, are so long as to keep the leaves 
quite remote from one another; and in it each leaf is almost 
symmetrically peltate, with a bilateralness indicated only by 
a seam over the line of the foot-stalk. The leaves of the 
Nymphea, Fig. 218, more closely clustered, and having less 


(SE (SE 217 218 
room te regis than longitudinally, exhibit a marked 


advance to the two-sided form; not only in the excess of 
the length over the breadth, but in the existence of a cleft, 


142 MORPHOLOGICAL DEVELOPMENT. 


where in the Victoria regia there is merely a seam. Among 
land-plants similar forms are found under analogous condi- 
tions. The common Hydrocotyle, Fig. 219, which sends 


up direct from its roots a few almost upright leaf-stalks, has 
these surmounted by peltate leaves; which leaves, however, 
diverge slightly from radial symmetry in correspondence with 
the slight contrast of circumstances which their grouping in- 
volves. Another case is supplied by the Nasturtewm, Fig. 
220, which combines the characters—a creeping stem, long 
leaf-stalks growing up at right angles to it, and unsymme- 
trically peltate leaves, of which the least dimension is, on 
the average, towards the stem. But perhaps the most 
striking illustration is that furnished by the Cotyledon umbi- 
licus, Fig. 221, in which different kinds of symmetry occur 
in the leaves of the same plant, along with differences in their 
relations to conditions. The root-leaves, a, that grow up on 
vertical petioles before the flower-stalk makes its appearance, 
are symmetrically peltate; while the leaves which subse- 
quently grow out of the flower-stalk, b, are at the bottom 
transitionally bilateral, and higher up completely bilateral. 
That the bilateral form of leaf is the ordinary form, 
corresponds with the fact that, ordinarily, the circum- 
stances of the leaf are different in the direction of the plant’s 
axis from what they are in the opposite direction, while — 


THE SHAPES OF LEAVES. 143 


transversely the circumstances are alike. It is needless to 
give diagrams to illustrate this extremely familiar truth. 
Whether they are broad or long, oval or heart-shaped, pointed 
or obtuse, the leaves of most trees and plants will be remem- 
bered by all as having the ends by which they are attached 
unlike the free ends, while the two sides are alike. And it will 
also be remembered that these equalities and inequalities of 
development correspond with the equalities and inequalities 
in the incidence of forces. 


§ 230. A confirmation that is interesting and important, 
is furnished by the cases in which leaves present unsymme- 
trical forms in positions where their parts are unsymmetri- 
cally related to the environment. A considerable deviation 
from bilateral symmetry may be seen in a leaf which habitu- 
ally so carries itself, that the half on the one side of the midrib 
is more shaded than the other half. The drooping branches of 
the Lime, exemplified in Fig. 222, show us leaves so arranged 


and so modified. On examining their attitudes and their 
relations one to another, it will be found that each leaf is so 
inclined that the half of it next the shoot grows over the 
shoot and gets plenty of light ; while the other half so hangs 
down that it comes a good deal into the shade of the pre- 
ceding leaf. The result is that having leaves which fall into 
these positions, the species profits by a large development of 
the exposed halves; and by survival of the fittest acting 
along with the direct effect of extra exposure, this modifi- 
cation becomes established. How unquestionable is the 
connexion between the relative positions of the halves and 
their relative developments, will be admitted on observing a 


144 MORPHOLOGICAL DEVELOPMENT. 


converse case. Fig. 223 represents a shoot of Goldfussia 
glomerata. Here the leaves are so set on the stem that the 
inner half of each leaf is shaded by the subsequently-formed 
leaf, while its outer half is not thus shaded; and here we find 
the inner half less developed than the outer half. But the 
most conclusive evidence of this relation between unsymme: 
trical form and unsymmetrical distribution of surrounding 
forces, is supplied by the genus Begonia ; for in it we have 
a manifest proportion between the degree of the alleged 
effect and the degree of the alleged cause. These plants 
produce their leaves in pairs, in such a way that the connate 
leaves interfere with one another, much or little according 
as the foot-stalks are short or long; and the result is a cor- 
relative divergence from symmetry. In Begonia nelumhae- 
Jolia, which has petioles so long that the connate leaves are not 
kept close together, there is but little deviation from a bilate- 
rally-peltate form; whereas, accompanying the compara- 
tively marked and constant proximity in B. pruinata, Fig. 
224, we see a more decidedly unsymmetrical shape ; and in 
B. mahringu, Fig. 225, the modification thus caused is 
pushed so far as to destroy the peltate structure. * 


§ 231. Again, then, we are taught the same truth. Here, 
as before, we see that homologous units of any order become 


* We may note that some of these leaves, as those of the Lime, furnish indica- 
tions of the ratio which exists between the effects of individual circumstances and 
those of typical tendencies. On the one hand, the leaves borne by these drooping 
branches of the Lime are with hardly an exception unsymmetrical more or less 
decidedly, even in positions where the causes of unsymmetry are not in action: a 
fact showing us the repetition of the type irrespective of the conditions, On the 
other hand, the degree of deviation from symmetry is extremely variable, even on 
the same shoot: a fact proving that the circumstances of the individual leaf are 
highly influential in modifying its form. But the most striking evidence of this 
direct modification is afforded by the suckers of the Lime. Growing, as these 
do, in approximately upright attitudes, the leaves they bear do not stand to one 
another in the way above described, and the causes of unsymmetry are not in 
action; and here, though there is a general leaning to the unsymmetrical form, 
a large proportion of the leaves become quite symmetrical. 


THE SHAPES OF LEAVES. 145 


differentiated in proportion as their relations to incident 
forces become different. And here, as before, we see that in 
each unit, considered by itself, the differences of dimension 
are greatest in those directions in which the parts are most 
differently conditioned; while there are no differences be- 
tween the dimensions of the parts that are not differently 
conditioned.* 


* It was by an observation on the forms of leaves, that I was first led to the 
views set forth in the preceding and succeeding chapters on the morphological 
differentiation of plants and animals. In the year 1851, during a country 
ramble in which the structures of plants had been a topic of conversation with a 
friend—Mr G. H. Lewes—I happened to pick up the leaf of a buttercup, and 
drawing it by its foot-stalk through my fingers so as to thrust together its deeply- 
cleft divisions, observed that its palmate and almost radial form was changed 
into a bilateral one; and that were the divisions to grow together in this new 
position, an ordinary bilateral leaf would result. Joining this observation with 
the familiar fact that leaves, in common with the larger members of plants, 
habitually turn themselves to the light, it occurred to me that a natural change 
in the circumstances of the leaf might readily cause such a modification of form ag 
that which I had produced artificially. If, as they often do with plants, soil 
and climate were greatly to change the habit of the buttercup, making it 
branched and shrub-like; and if these palmate leaves were thus much over- 
shadowed by each other; would not the inner segments of the leaves grow 
towards the periphery of the plant where the light was greatest, and so change 
the palmate form into a more decidedly bilateral form? Immediately I began to 
look round for evidence of the relation between the forms of leaves and the general 
characters of the plants they belonged to; and soon found some signs of con- 
nexion. Certain anomalies, or seeming anomalies, however, prevented me from 
then pursuing the inquiry much further. But consideration cleared up these 
difficulties; and the idea afterwards widened into the general doctrine here 
elaborated. Occupation with other things prevented me from giving expression 
to this general doctrine until Jan. 1859; when i published an outline of it in 
the Medico-Chirurgical Review. 


VOL, If. nN 


CHAPTER X. 
THE SHAPES OF FLOWERS. 


§ 232. FoLttow1ne an order like that of preceding chap- 
rers, let us first note a few typical facts respecting the forms 
of clusters of flowers, apart from the forms of the flowers them- 
selves. Two kindred kinds of Leguminose will serve to show 
how the membersof clusters are distributed in an all-sided man- 
ner or in a two-sided manner, according as the circumstances 
are alike on all sides or alike on only two sides. In Aippo- 
crepis, represented in Fig. 226, the flowers growing at the end 
of a vertical stem, are arranged 
round it in radial symmetry. 
Contrariwise in Melilotus, Fig. 
WE, 227, where the axillary stem 
~/ bearing the flowers is so 
ge-7 placed in relation to the main 
stem, that its outer and inner 
sides are differently condi- 
tioned, the flowers are all on 
the outer side: the cluster is 
bilaterally symmetrical, since 
it may be cut into approx- 
imately equal and similar 
groups by a vertical plane passing through the main axis. 

Plants of this same tribe furnish clusters of intermediate 
characters having intermediate conditions. Among these, 
as among the clusters which other types present, may be 


THE SHAPES OF FLOWERS. 147 


found some in which conformity to the general law is not 
obvious. The discussion of these apparent anomalies would 
carry us too much out of our course. A clue to the explana- 
tion of them will, I believe, be found in the explanation 
presently to be given of certain kindred anomalies in the 
forms of individual flowers. 


§ 233. The radially-symmetrical form is common to all 
individual flowers that have vertical axes. In plants which 
are practically if not literally uniaxial, and bear their flowers 
at the ends of upright stalks, so that the faces open hori- 
zontally, the petals are disposed in an all-sided way. Cro- 
cuses, Tulips, and Poppies are familiar examples of this struc- 
ture occurring under these conditions. A Ranunculus flower, 
Fig. 228, will serve as a typical one. Similarly, flowers 
which have peduncles flexible enough to 
let them hang directly downwards, and 
are not laterally incommoded, are also 

radial; as in the Fuchsia, Fig. 229, as 
“in Cyclamen, Hyacinth, &c. These rela- 
tions of form to position are, I believe, 
uniform. Though some flowers carried at the ends of up- 
right or downright stems have oblique shapes, it is only when 
they have inclined axes or are not equally conditioned all 
round. No solitary flower having an axis habitually ver- 
tical, presents a bilateral form. This is as we should expect, 
since flowers which open out their faces horizontally, 
whether facing upwards or downwards, are, on the average, 
similarly affected on all sides. 

At first it seems that flowers thus placed should alone 
be radial ; but further consideration discloses conditions under 
which this type of symmetry may exist in flowers otherwise 
placed. Remembering that the radial form is the primitive 
form—that, morphologically speaking, it results from the 
contraction into a whorl, of parts that are originally arranged 
in the same spiral succession as the leaves; we must expect 

10 * 


148 MORPHOLOGICAL’ DEVELOPMENT. 


it to continue wherever there:are no forces tending to change 
it. What now must be the forces tending to change it? 
They must be forces which do not simply affect differently 
the different parts of an individual flower ; but they must be 
forces which affect in like contrasted ways the homologous 
parts of other individual flowers, both on the same plant and 
on surrounding plants of the same species. A permanent 
modification can be expected only in cases where, by inherit- 
ance, the effect of the modifying causes accumulates. That 
it may accumulate the flowers must keep themselves so re- 
lated to the environment, that the homologous parts may 
generation after generation be subjected to like differentiating 
forces. Hence, among a plant’s flowers which maintain no uni- 
formity in the relations of their parts to surrounding influences, ~ 
the radial form will continue. let us glance at the several 
causes which entail this variability. When flowers 
are borne on many branches, which have all inclinations from 
the vertical to the horizontal—as are the flowers of the Apple, 
the Plum, the Hawthorn—they are placed in countless different 
attitudes. Consequently, any spontaneous variation in shape 
which might be advantageous were the attitude constant, is 
not likely to be advantageous ; and any functionally-produced 
modification in one flower is likely to be neutralized in off- 
spring by some opposite functionally-produced modification 
in another flower. It is quite comprehensible, therefore, 
that irregularly-branched plants should thus preserve their 
laterally-borne flowers from undergoing permanent devia- 
tions from their primitive radial symmetry. Fig. 230, re- 
ys presenting a blossoming 

twig of the Blackthorn, 
will illustrate this. 
Again, upright panicles 
such as that of the 
\ Saxifrage, shown in Fig. 

281, and irregular terminal groups of flowers otherwise 
named, furnish conditions under which there is similarly an 


THE SHAPES OF FLOWERS. 149 


absence of determinate relations between the parts of the 
flowers and the incident forces; and hence an absence of 
bilateralness. This inconstancy of relative position 
is produced in various other ways—by extreme flexibility of 
the peduncles, as in the Blue-bell; by the tendency of the 
peduncles to curl to a greater or less extent in different 
directions, as in Pyrola; by special twisting of the peduncles, 
‘differing in degree in different individuals, as in Convol- 
vulus ; by extreme flexibility of the petals, as in Lythrum. 
Elsewhere the like general result arises from a progressive 
change of attitude; as in Myosotis, the stem of which as it 
unfolds causes each flower to undergo a transition from an 
upward position of the mouth to a lateral position ; or as in 
most Crucifere, where the like effect follows from an altered 
direction of the peduncle. 

There are, however, certain seemingly anomalous cases 
where radial sympathy is maintained by laterally-placed 
flowers, which keep their parts in relative positions that are 
tolerably constant. The explanation of these exceptions is 
not manifest. It is only when we take into account certain 
incident actions liable to be left unremembered, that we find 
a probable solution. It will be most convenient to postpone 
the consideration of these cases until we have reached the 
general rule to which they are exceptions. 


§ 234. Transitions varying in degree from the radial to- 
wards the bilateral, are common in flowers that are borne at 
the ends of branches or axes which are inclined in tolerably 
constant ways. We may see this in sundry garden flowers 
such as Petunia, or such as Tydea and Achimenes shown in 
253 Figs. 232 and 233. If these 

plants be examined, it will 

—O x be perceived that the mode 

A y\) of growth makes the flower 


unfold in a partially one- 
sided position; that its parts of attachment have rigidity 


150 MORPHOLOGICAL DEVELOPMENT. 


sufficient to prevent this attitude from being very much 
interfered with ; and that though the individual flowers vary 
somewhat in their attitudes, they do not vary to the extent of 
neutralizing the differentiating conditions—there remains an 
average divergence from a horizontal unfolding of the flower, 
to account for its divergence from radial symmetry. 

We pass insensibly from forms like these, to forms having 
bilateral symmetry strongly pronounced. Some such forms 
occur among flowers that grow at the ends of upright stems ; 
asin Pinguicula, and in the Violet tribe. But this happens 
only where in successive generations the flower unfolds its 
parts sideways in constant relative positions. And in the 
immense majority of flowers that have well-marked two-sided 
forms, the habitual exposure of the different parts to different 
sets of forces, is effectually secured by the mode of placing. 
As illustrations I may name the genera—Orchis, Utricularia, 
Salvia, Salix, Delphinum, Mentha, Teucriwm, Ajuga, Badllota, 
Galeopsis, Lamium, Stachys, Glechoma, Marrubium, Cala- 
mintha, Chnopodium, Melittis, Prunella, Scutellaria, Bart- 
sia, Euphrasia, Rhinanthus, Melampyrum, Pedicularis, Lan- 
arta, Digitalis, Orobanche, Fumaria, §c. ; to which may be 
added, all the Grasses and all the Papilionacew. In most of 
these cases the flowers, being sessile on the sides of upright 
stems, are kept in quite fixed attitudes; and in the other 
cases the peduncles are very short, or else stiff enough to 
secure general uniformity in the positions. <A few of the 
more marked types are shown in Figs. 234 to 241. 


B35 “1238 \ 2376256 


rs a 


Very instructive evidences here meet us. Sometimes with- 
in the limits of one genus we find radial flowers, bilateral 
flowers, and flowers of intermediate characters. The genus 
Begonia may be instanced. In B. rigida the flowers, various 


THE SHAPES OF FLOWERS. 151 


in their attitudes, are in their more conspicuous characters 
radial: though there is a certain bilateralness in the calyx, 
the five petals are symmetrically disposed all round. B. 
Wagneriana furnishes two forms of flowers: on the same in- 
dividual plant may be found radial flowers like Fig. 242, and 
others like Fig. 243 that are merging into the bilateral. 
More decided is the bilateralness in B. albo-coccinea, Fig. 244 ; 
and still more in B. nitida, Fig. 245. While in B. jatrophe- 


foe OR 


folia, Fig. 246, the change reaches its extreme by the dis- 
appearance of the lateral petals. On examining the modes of 
growth in these several species, they will be seen to explain 
these changes in the manner alleged. Even 
more conclusive are the nearly-allied transformations occur- 
ring in artificially-produced varieties of the same species. 
Gloxinia may be named in illustration. In Fig. 247 is repre- 
sented one of the ordinary forms, which shows us bilateralness of 
shape along with a mode of growth that renders the conditions 
alike on the two sides while different above and below. But 
in G. erecta, Fig. 248, we | 


\ 247. 248 
have the flower assuming an 


: <n 
A " 


upright attitude, and at the 
same time assuming the radial 
type. This is not to be inter- 
preted as a production of ra- 
dial symmetry out of bilateral symmetry, under the action of 
the appropriate conditions. It is rather to be taken as a case 
of what is termed “ peloria ”’—a reversion to the primitive 
radial type, from which the bilateral modification had heen 
derived. The significant inference to be drawn from it is, 
that this primitive radial type had an upright attitude ; 1nd 


152 MORPHOLOGICAL DEVELOPMENT. 


that the derivation of a bilateral type from it, occurred along 
with the assumption of an inclined attitude. 

We come now to a group of cases above referred to, in 
which radial symmetry continues to co-exist with that con- 
stant lateral attitude ordinarily accompanied by the two-sided 
form. Two examples will suffice: one a very large flower, 
the Hollyhock, and the other a very small flower, the Agri- 
mony. Why does the radial form here remain unchanged ? 
and how does its continuance consist with the alleged general 
law ? 

Until quite recently I have been unable to find any pro- 
bable answers to these questions. When the difficulty first 
presented itself, I could think of no other possible cause for 
the anomaly, than that the parts of the Hollyhock-flower, 
unfolding spirally as they do, might have different degrees of 
spiral twist in different flowers, and might thus not be unfolded. 
in sufficiently-constant positions. But this seemed a very 
questionable interpretation ; and one which did not obviously 
apply to the case of the Agrimony. It was only on inquiring 
what are the special causes of modifications in the forms of 
flowers, that a more feasible explanation suggested itself; and 
this would probably never have suggested itself, had not Mr 
Darwin’s investigations into the fertilization of Orchids led 
me to take into account an unnoticed agency. 

The actions which affect the forms of leaves, affect much 
less decidedly the forms of flowers; and the forms of flowers 
are influenced by actions that do not influence the forms of 
leaves. Partly through the direct action of incident forces 
and partly through the indirect action of natural selection, 
leaves get their parts distributed in ways that most facilitate 
their assimilative functions, under the circumstances in which 
they are placed ; and their several types of symmetry are thus 
explicable. But in flowers, the petals and fructifying organs 
of which do not contain chlorophyll, the tendency to grow 
most where the supply of light is greatest, is less decided, if 
not absent; and a shape otherwise determined is hence less 


THE SHAPES OF FLOWERS. 153 


liable to alter in consequence of altered relations to sun and 
air. Gravity, too, must be comparatively ineffective in caus- 
ing modifications: the smaller sizes of the parts, as well as 
their modes of attachment, giving them greater relative 
rigidity. Not, indeed, that these incident forces of the inor- 
ganic world are here quite inoperative. Fig. 2+9 
249, representing a species of Campanula, 
shows that the developments of individual flow- 
ers are somewhat modified by the relations of 
their parts to general conditions. But the 
fact to be observed is, that the extreme trans- 
formations which flowers undergo are not 
likely to be thus caused: some further cause 
must be sought. And if we bear in mind 
the feettons | of flowers, we shall find in their 
adaptations to their functions, under Gsuiiitions that are 
extremely varied, an adequate cause for the different types 
of symmetry, as well as for the exceptions to them. Flow- 
ers are parts in which fertilization is effected; and the 
active agents of this fertilization are eee moths, 
butterflies, &. Mr Darwin has shown in many cases, that 
the forms and positions of the essential organs of fructifica- 
tion, are such as to facilitate the actions of insects in trans- 
ferring pollen from the anthers of one flower to the pistil of 
another— an arrangement produced by natural selection. 
And here we shall find reason for concluding, that the forms 
and positions of those subsidiary parts which give the gene- 
ral shape to the flower, similarly arise by the survival of 
individuals which have the subsidiary parts so adjusted as to 
aid this fertilizing process—the deviations from radial sym- 
metry being among such adjustments. The reasoning is as 
follows. So long as the axis of a flower is vertical and 
the conditions are similar all round, a bee or butterfly alight- 
ing on it, will be as likely to come from one side as from 
another ; and hence, hindrance rather than facilitation would 
result if the several sides of the flower did not afford it equally 


154 MORPHOLOGICAL DEVELOPMENT, 


free access. In like manner, flowers which are distributed 
over a plant in such ways that their discs open out on 
planes of all directions and inclinations, will have no tend- 
ency to lose their radial symmetry; since, on the average, 
no part of the periphery is differently related to insect- 
agency from any other part. But flowers so fixed as to 
open out sideways in tolerably-constant attitudes, have 
their petals differently related to insect-agency. A bee or 
butterfly coming to a laterally-growing flower, does not set- 
tle on it in one way as readily as in another; but almost of 
necessity settles with the axis of its body inclined upwards 
towards the stem of the plant. Hence, the side-petals of a 
flower so fixed, habitually stand to the alighting insect in 
relations different from those in which the upper and lower 
petals stand; and the upper and lower petals differ from one 
another in their relations to it. If, then, there so arises an 
habitual attitude of the insect towards the petals, there must 
be some particular arrangement of the petals that will be 
most convenient to the insect—will most facilitate its en- 
trance into the flower. Thus we see in many cases, that a 
long undermost petal or lip, by enabling the insect to settle 
in such way as to bring its head opposite to the opening of 
the tube, aids its fertilizing agency. But whatever be the spe- 
cial modifications of the corolla which facilitate the actions of 
_ the particular insects concerned, all of them will conduce to 
bilateral symmetry ; since they will be alike for the two sides 
but unlike for the top and bottom. And now we 
are prepared for understanding the exceptions. Flowers 
growing sideways can become thus adapted by survival of 
the fittest, only if they are of such sizes and structures that 
insect-agency can affect them in the way described. But 
in the plants named above, this condition is not fulfilled. A 
Hollyhock-flower is so open, as well as so large, that its petals 
are not in any appreciable degree differently related to the. 
insects which visit it. On the other hand, the flower of the 
Agrimony is so small, that unless visited by insects of a 


THE SHAPES OF FLOWERS. 155 


corresponding size which settle as bees and butterflies settle, 
its parts will not be affected in the alleged manner. That 
all anomalies of this kind can at once be satisfactorily ex- 
plained, is scarcely to be expected: the circumstances of 
each case have to be studied. But it seems not improbable 
that they are all due to causes of the kind indicated. 


§ 235. We nave already glanced at clusters of flowers 
for the purpose of considering their shapes as clusters. We 
must now return to them to observe the modifications under- 
gone by their component flowers. Among these occur illus- 
trations of great significance. 

An example of transition from the radial'to the bilateral 
form in clustered flowers of the same-species, is furnished by 
the cultivated Geraniums, called by florists Pelargoniums, 
Some of these bearing somewhat small terminal clusters 
of flowers, which are closely packed together, with their 
faces almost upwards, have radially-symmetrical flowers. 
But among other varieties having terminal clusters of which 
the members are mutually thrust on one side by crowding, 
the flowers depart very considerably from the radial shape 
towards the bilateral shape. A like result occurs under 
like conditions in Rhododendrons and Azaleas. The Verbena, 
too, furnishes an illustration of radial flowers rendered 
slightly two-sided by the.slight two-sideness of their rela- 
tions to other flowers in the cluster. And among the Cruci- 
Jere, a kindred case occurs in the cultivated Candytutft. 

Evidence of a somewhat different kind, is offered us by 
clustered flowers in which the peripheral members of the 
clusters differ from the central members; and this evidence 
is especially conclusive where we find allied species that do 
not exhibit the deviation, at the same time that they do not 
fulfil the conditions under which it may be expected. Thus, 
in Scabiosa succisa, Fig. 250, which bears its numerous small 
flowers in a hemispherical knob, the component flowers, 
similarly circumstanced, are all equal and all radial; but in 


156 MORPHOLOGICAL DEVELOPMENT. 


Scabiosa arvensis, Fig. 251, in which the numerous small 
flowers form a flattened disk, 
only the confined central ones. 
are radial: round the edge the 
flowers are much larger, and 
conspicuously bilateral. 

But the most remarkable 
and most conclusive proofs of these relations between forms 
and, positions, are those given by the clustered flowers called 
Umbellifere. In some cases, as where the component flowers 
have all plenty of room, or where the surface of the umbel is 
more or less globular, the modifications are not conspicuous ; 
but where, asin Viburnum, Cherophyllum, Anthriseus, Torihs, 
Caucalis, Daucus, Tordylium, &c., we have flowers clustered 
in such ways as to be differently conditioned, we find a num- 
ber of modifications that are marked and varied in propor- 
tion as the differences of conditions are marked and varied. 
In Cherophyllum, where the flowers of each umbellule are 
closely placed so as to form a flat surface, but where the 
umbellules are wide apart and form a dispersed umbel, the 
umbellules do not differ from one another ; though among the 
flowers of each umbellule there are decided differences—the 
central flowers being small and radial, while the peripheral 
ones are large and bilateral. But in other genera, where not 
only the flowers of each umbellule but also the umbellules 
themselves are closely clustered into a flat surface, the umbel- 
lules themselves become contrasted; and many remarkable 
secondary modifications arise. In an umbel of Heraclewm, 
for instance, there are to be noted the facts :—first, that the. 
external umbellules are larger than the internal ones; 
second, that in each umbellule the central flowers are less 
developed than the peripheral ones ; third, that this greater 
development of the peripheral flowers is most marked in the 
outer umbellules; fourth, that it is most marked on the outer 
sides of the outer umbellules; fifth, that while the interior 
flowers of each umbellule are radial, the exterior ones are 


THE SHAPES OF FLOWERS. 15Y 


bilateral; sixth, that this bilateralness 1s most marked «in 
the peripheral flowers of the peripheral umbellules; seventh, 
that the flowers on the outer side of these peripheral 
aumbellules are those in which the bilateralness reaches a 
maximum; and eighth, that where the outer umbellules 
touch each other, the flowers, being unsymmetrically 
‘placed, are unsymmetrically bilateral.* The like modi- 
fications are displayed, though not in so clearly-trace- 
-able a way, in an umbel of Zordylium, Fig. 252. Considering 
-how obviously these various 
forms are related to the vari- 
-ous conditions, we should be 
-scarcely able, even in the 
absence of all other facts, to 
resist the conclusion that the 
differences in the conditions 
are the causes of the differ- 
ences in the forms. 
Composite flowers furnish 
-evidence so nearly allied to 
‘that which clustered flowers 
furnish, that we may fitly glance at them under the same 
‘head. Such a common type of this order as the Sun-fiower, 
exemplifies the extremely marked differ- , 
-ence that arises in many of these plants 
between the closely-packed internal 
florets, each similarly circumstanced on 
-all sides, and the external florets, not 
similarly circumstanced on all sides. 
In Fig. 253, representing the inner and 
outer florets of a Daisy, the contrast is 
'marked between the small radial corolla of the one and the 
larger bilateral corolla of the other. In many cases, how- 
ever, this contrast is less marked: the inner florets having 


* TJ had intended here to insert a figure exhibiting these differences; butas the 
- Cow-parsnip does not flower till July, and as I can find no drawing of the umbel 
«which adequately represents its details, I am obliged to take another instance. 


158 MORPHOLOGICAL DEVELOPMENT. 


also their outward-growing prolongations—a difference pos- 
sibly related to some difference in the habits of the insects 
that fertilize them. Nevertheless, these composite flowers 
which have inner florets with strap-shaped corollas out- 
wardly directed, equally conform to the general principle; 
both in the radial arrangement of the assemblage of florets, 
and in the bilateral shape of each floret; which has its 
parts alike on the two sides of a line passing from the centre 
of the assemblage to the circumference. Certain 
other members of this order fulfil the law somewhat differ- 
ently. In Centaurea, for instance, the inner florets are small 
and vertical in direction, while the outer florets are large and 
lateral in direction. And here may be remarked, in passing, 
a clear indication of the effect which great flexibility of the 
petals has in preventing a flower from losing its original 
radiate form ; for while in C. cyanus, the large outward-grow- 
ing florets, having short, stiff divisions of the corolla, are 
decidedly bilateral, in C. scabiosa, where the divisions of the 
corolla are long and flexible, the radial form is scarcely at 
all modified. On bearing in mind the probable relations of 
the forms to insect-agency, the meaning of this difference 
will not be difficult to understand. 


§ 236. In extremely-varied ways there are thus re-illus- 
trated among flowers, the general laws of form which leaves 
and branches and entire plants disclose to us. Composed as 
each cluster of flowers is of individuals that are originally 
similar ; and composed as each flower is of homologous foliar 
organs; we see both that the like flowers become unlike and 
the like parts of each flower become unlike, where the posi- 
tions. involve unlike incidence of forces. The symmetry 
remains radial where the conditions are equal all round; 
shows deviation towards two-sidedness where there is slight 
two-sidedness of conditions; becomes decidedly bilateral 
where the conditions are decidedly bilateral; and passes into 
an unsymmetrical form where the relations to the environ- 
ment are unsymmetrical. 


CHAPTER XI. 
THE SHAPES OF VEGETAL CELLS. 


§ 237. Wer come now to aggregates of the lowest order. 
Already something has been said (§ 217) concerning the 
forms of those morphological units which exist as independent 
plants. But it is here requisite briefly to note the modifica- 
tions undergone by them where they become components of 
larger plants. 

Of the numerous cell-forms which are found in the tissues 
of the higher plants, it will suffice to give, in Fig. 254, re- 
presenting a section cf the surface of 
a leaf, a single example. In this it 
will be seen that the epidermis cells 
ce, covered by the secreted external 
layer a, and separated from the layer 
of cells below them by the masses of 
inter-cellular substance 6, have differ- 
entiations of form clearly related to 
differences in the incidence of forces. Their divergences from 
primordial sphericity are such as correspond with the un- 
likenesses in the circumstances of their respective sides. 
Similarly with the layers below them. And throughout the 
more complex modifications which the cells of other tissues 
exhibit, the like correspondence holds. 

Among plants of a lower order of aggregation, we have al- 
ready seen how cells become metamorphosed as they become 
integrated into masses having definite organizations. The 


160 MORPHOLOGICAL DEVELOPMENT. 


higher Alge, exemplified in Figs. 82, 34, 35, show this very 


clearly. Here the departure from 
the simple cell-form to the form of 
an elongated prism, is manifestly 
Subordinated to the contrasts in the 
pee S relations of the parts. And it is 
interesting to observe how, in one 
of the branches of Fig. 32, we pass 
from the small, almost-spherical 
cells which terminate the branch- 
lets, to the large, much-modified 
a cells which jom the main stem, 
through np as obviously related in their changed 
forms to the altered actions their positions expose them to. 
More simply, but quite as conclusively, do the inferior 
Alge, of which Figs. 19—23 are examples, show us how 


cells pass from their original spherical symmetry into radial 
symmetry, as they pass from a state in which they are simi- 
larly-conditioned on all sides, to a state in which two of their 
opposite sides or ends are conditioned in ways that are like 
one another, but unlike the ways in which all other sides are 
conditioned. 

Still more instructive are the morphological differentiations 
of those protophytes in which the first steps towards a higher 
degree of integration are shown. Fig. 9 represents one of 
the transitional forms of Desmidtacee. In it we see that the 
two inner halves by which the individuals are united, differ 


THE SHAPES OF VEGETAL CELLS. 161 


somewhat from the two outer halves. So, too, of the type 
exemplified by Fig. 10, it is to be noted that besides the 
difference between the transverse and longitudinal dimensions, 
which the component units display in common, the two end 
units differ from the rest: they have appendages which the 


rest have not. Once more, where the integration is car- 
ried on in such ways as to produce not strings but clusters, 
there arise contrasts and correspondences just such as might 
be looked for. All the four members of the group shown in 
Fig. 12, are similarly conditioned ; and each of them has 
a bilateral shape answering to its bilateral relations. In 
Fig. 14 we have a number of similarly-bilateral individuals 
on the circumference, including a central individual differing 
from the rest by having the bilateral character nearly 
obliterated. And then, in Fig. 15, we have two central 
components of the group, deviating more decidedly from 
those that surround them. 

These few typical facts, which must be taken like the few 
typical facts grouped in each of the foregoing chapters as 
indicating a mass of evidence too great to be here detailed, 
will sufficiently show that from the most complex vegetal 
types down to the most simple, the laws of morphological 
differentiation remain the same. 


VOu iL. ll 


CHAPTER XII, 
CHANGES OF SHAPE OTHERWISE CAUSED. 


§ 238. Brstpzs the more special causes of modification in 
the shapes of plants and of their parts, certain more general 
causes must be briefly noticed. These may be described as 
consequences of variations in the total quantities of the 
matters and forces furnished to plants by their environments. 
Some of the changes of form so produced are displayed by 
plants as wholes, and others only by their parts. We will 
glance at them in this order. 


§ 239. It is a familiar fact that luxuriant shoots have re- 
latively-long internodes; and, conversely, that a shoot 
dwarfed trom lack of sap, has its nodes closely clustered : the 
result being that the lateral axes, where these are developed, 
become in the one case far apart and in the other case near 
together. Fig. 255 represents a branch to the parts of which 
thelongerand shorter internodes so result- 
ing give differential characters. A whole 

) tree being in many cases simultaneously 
thus affected by states of the earth or the 
air, all parts of it may have such varia- 
tions impressed on them; and, indeed, such 
variations, following more or less regu- 
larly the changes of the seasons, give to 

; many trees manifest traits of structure. 


In Fig. 256, a shoot of Phyllocactius 


aI IO 


CHANGES OF SHAPE OTHERWISE CAUSED. 163 


crenatus, we have an interesting example of a variation essen- 
tially of the same nature, little as it appears to be so. For 
each of the lateral indentations is here the seat of an axillary 
bud; and these we see are separated by internodes which, 
becoming broader as they become longer, and narrower as 
they become shorter, produce changes of form that correspond 
with changes in the luxuriance of growth. 

To complete the statement it must be added that these 
variations of nutrition often determine the development or 
non-development of lateral axes ; and by so doing cause still 
more marked structural differences. The Fox-glove may be 
named as a plant which illustrates this truth. 


§ 240. From the morphological differentiations caused by 
unlikenesses of nutrition which the whole plant feels, we pass 
now to those which are thus caused in some of its parts and 
not in others. Among such are the contrasts between 
flowering axes, and the axes that bear leaves only. It has 
already been shown in § 78, that the belief expressed by 
Wolff in a direct connexion between fructification and innu- 
trition, is justified inductively by many facts of many kinds. 
Deductively too, in § 79, we saw reason to conclude that such 
a relation would be established by survival of the fittest ; 
seeing that it would profit a species for its members to begin 
sending off migrating germs from the ends of those axes 
which innutrition prevented from further agamogenetic mul- 
tiplication. Once more, when considering the nature of the 
phznogamic axis, we found support for this belief in the fact 
that the components of a flower exhibit a reversion to that 
type from which the phznogamic type has probably arisen— 
a reversion which the laws of embryology would lead us to 
look for where innutrition had arrested development. 

Hence, then, we may properly count those deviations of 
structure which constitute inflorescence, as among the mor- 
phological differentiations produced by local innutrition. Ido 


not mean that the detailed modifications which the essential 
11 * 


164 MORPHOLOGICAL DEVELOPMENT 


and subservient organs of fructification display, are thus 
accounted for: we have seen reason to think them otherwise 
caused. But I mean that the morphological characters which 
distinguish gamogenetic axes in general from agamogenetic 
axes, such as non-development of the internodes, and dwarf- 
ing of the foliar organs, are primarily results of failure in 
the supply of some material required for further growth.* 


§ 241. Another trait which has to be noticed under this 
head, is the spiral, or rather the helical, arrangement of 
parts. The successive nodes of a phenogam habitually bear 
their appendages in ways implying more or less twist in the 
substance of the axis; and in climbing plants the twist is such 
as to produce a corkscrew shape. ‘This structure is ascribable 
to differences of interstitial nutrition. Taking a shoot that 
is growing vertically, it is clear that if the molecules are 
added with perfect equality on all sides, there will be no 
tendency towards any kind of lateral deviation; and the 
successively-produced parts will be perpendicularly over one 


* It is but just to the memory of Wolff, here to point out that he was im- 
mensely in advance of Goethe in his rationale of these metamorphoses. Whatever 
greater elaboration Goethe gave to the theory considered as an induction, seems 
to me more than counter-balanced by the irrationality of his deductive interpret- 
ation; which unites medieval physiology with Platonic philosophy. A domin- 
ant idea with him is that leaves exist for the purpose of carrying off crude juices— 
that ‘‘as long as there are crude juices to be carried off, the plant must be pro- 
vided with organs competent to effect the task ;” that while “ the less pure fluids 
are got rid of, purer ones are introduced ;” and that “if nourishment is withheld, 
that operation of nature (flowering) is facilitated and hastened; the organs of the 
nodes (leaves) become more refined in texture, the action of the purified juices 
becomes stronger, and the transformation of parts having now become possible, 
takes place without delay.” This being the proximate explanation, the ultimate 
explanation is, that Nature wishes to form flowers—that when a plant flowers it 
‘attains the end prescribed to it by nature ;” and that so “nature at length at- 
tains her object.”” Instead of vitiating his induction by a teleology that is as 
unwarranted in its assigned object as in its assigned means, Wolff ascribes the 
phenomena to a cause which, whether sufficient or not, is strictly scientific in 
its character. Variation of nutrition is unquestionably a “true cause’ of vari- 
ation in plant-structure. We have hereno imaginary action of a fictitious agency ; 
but an ascertained action of a known agency. 


CHANGES OF SHAPE OTHERWISE CAUSED. 165 


another. But any inequality in the rate of growth on the 
different sides of the shoot, will destroy this straightness in 
the lines of growth. If the greatest and least rates of mole- 
cular increase happens to be on opposite sides, the shoot must 
assume a curve of single curvature ; but in every other case 
of unequal molecular increase, a curve of double curvature 
will result. Now it is a corollary from the instability of the 
homogeneous, that the rates of growth on all sides of a shoot 
can never be exactly alike ; and it is to be also inferred from 
thesame general law, that the greatestand least rates of growth 
will not occur on exactly opposite sides of the shoot, at the 
same time that equal rates of growth are preserved by the 
two other sides. Hence, there must almost inevitably arise 
more or less of twist; and the appendages of the internodes 
will so be prevented from occurring perpendicularly one over 
another. 

A deviation of this kind, necessarily initiated by physical 
causes in conformity with the general laws of evolution, is 
likely to be madé.regular and decided by natural selection. 
For under ordinary circumstances, a plant will profit by hay- 
ing its axis so twisted as to bring the appended leaves into 
positions that prevent them from shading oneanother. And, 
manifestly, modifications in the forms, sizes, and insertions 
of the leaves, may, under the same agency, lead to adapted 
modifications of the twist. We must therefore ascribe this 
common characteristic of phenogams, primarily to local differ- 
ences of nutrition, and secondarily to survival of the fittest. 

It is proper to add that there are some Monocotyledons, 
as Urania speciosa, in which this character does not occur. 
What conditions of existence they are that here hold this 
natural tendency in check, it is not easy to see.* 

* The Natural History Review for July, 1865, contained an article on the doe- 
trine of morphological composition set forth in the foregoing Chaps. I. to IIT. In 
this article, which unites exposition and criticism in a way that is unhappily not 
common with reviewers, it is suggested that the spiral structure may be caused 


by natural selection. When this article appeared, the foregoing five pages were 
standing over in type, as surplus from No. 14, issued in June, 1865. 


CHAPTER XIII. 
MORPHOLOGICAL DIFFERENTIATION IN ANIMALS. 


§ 242. Tue general considerations which preluded our in- 
quiry into the shapes of plants and their parts, equally serve, 
so far as they go, to prelude an inquiry into the shapes of 
animals and their parts. Among animals, as among plants, 
the formation of aggregates greater in bulk or higher in 
degree of composition, or both, is accompanied by changes of 
form in the aggregates as wholes as well as by changes of 
form in their parts; and the processes of morphological 
differentiation conform to the same general laws in the one 
kingdom as in the other. 

It is needless to recapitulate the several kinds of modifi- 
cation to be explained, and the several factors that co- 
operate in working them. In so far as these are common 
to plants and animals, the preceding chapters have suf- 
ficiently familiarized them. Nor is it needful to specify 
afresh the several types of symmetry and their descriptive 
names; for what is true of them in the one case is true of 
them in the other. There is, however, one new and all- 
important factor which we shall have now to take into 
account; and about this a few preliminary remarks are 
requisite. 


§ 243. This new factor is motion—motion of the organism 
in relation to surrounding objects, or of the parts of the 


MORPHOLOGICAL DIFFERENTIATION IN ANIMALS. 167 


organism in relation to one another, or both. Though there 
are plants, especially of the simpler kinds, which move, 
and though a few of the simpler animals do not move; yet 
movements are so exceptional and unobtrusive in the one 
kingdom, while they are so general and conspicuous in the 
other, that the broad distinction commonly made is well 
warranted. What, among plants, is an inappreciable cause 
of morphological differentiation, becomes, among animals, the 
chief cause of morphological differentiation. 

Animals that are rooted or otherwise fixed, of course present 
traits of structure nearest akin to those we have been lately 
studying. The motions of parts in relation to one another 
and to the environment, being governed by the mode of aggre- 
gation and mode of fixing, we are presented with morphological 
differentiations similar in their general characters to those of 
plants, and showing us parallel kinds of symmetry under 
parallel conditions. But animals which move from place 
to place are subject to an additional class of actions and re- 
actions. These actions and reactions affect them in various 
ways according to their various modes of movement. Let us 
glance at the several leading relations between shape and 
motion which we may expect to find. 

If an organism advances through a homogeneous medium 
with one end always foremost, that end, being exposed to 
forces unlike those to which the other end is exposed, may 
be expected to become unlike it; and supposing this to be 
the only constant contrast of conditions, we may expect an 
equal distribution of the parts round the axis of move- 
ment—a radial symmetry. If in addition to this 
habitual attitude of the ends, one surface of the body is 
always uppermost and another always lowermost, there arise 
between the top and bottom dissimilarities of conditions, 
while the two sides remain similarly conditioned. Hence it 
is inferable that such an organism will be divisible into 
similar halves by a vertical plane passing through its axis of 
motion—will have a bilateral symmetry. We may presume 


168 MORPHOLOGICAL DEVELOPMENT. 


that this symmetry will deviate but little from double 
bilateralness where the upper and under paxts are not exposed 
to strongly-contrasted influences; while we may rationally 
look for single bilateral symmetry of a decided kind, in 
creatures having dorsal and ventral parts conversant with 
very unlike regions of the environment: as in all cases 
where the movement is over a solid surface. If the 
movement, though over a solid surface, is not constant in 
direction, but takes place as often on one side as on another, 
radial symmetry may be again looked for; and if the motions 
are still more variously directed—if they are not limited to 
approximately-plane surfaces, but extend to surfaces that are 
distributed all around with a regular irregularity—an ap- 
proach of the radial towards the spherical symmetry is to be 
anticipated. Where the habits are such that the 
intercourse between the organism and its environment, does 
not involve an average equality of actions and reactions on 
any two or more sides, there may be expected either total 
irregularity or some divergence from regularity. 

The lke general relations between forms and incident 
forces are inferable in the component parts of animals, as 
well as in the animals as wholes. It is needless, however, to 
occupy space by descriptions of these. Let us now pass to 
the facts, and see how they confirm, a posteriori, the con- 
clusions here reached @ priori. 


CHAPTER XIV. 
THE GENERAL SHAPES OF ANIMALS. 


§ 244. Certain of the Protozoa are quite indefinite in 
their shapes, and quite inconstant in those indefinite shapes 
which they have—the relations of their parts are indeter- 
minate both in space and time. In one of the simpler 
Rhizopods, at least during the active stage of its existence, 
no permanent distinction of inside and outside is established ; 
and hence there can arise no established correspondence 
between the shape of the outside and the distribution of 
environing actions. But when the relation of mner and 
outer becomes fixed, either over part of the mass or over the 
whole of it, we have kinds of symmetry that correspond 
with the habitual incidence of forces. An Ameba in be- 
coming encysted, which we may regard as the production 
in it of a differentiation between superficial parts and central 
parts, passes from an indefinite, ever-changing form into 
a spherical form ; and the order of symmetry which it thus 
assumes, is in harmony with the average equality of the 
actions on all its sides. In Difflugia, Fig. 184, and still 
better in Arcella, we have an indefinitely-radial symmetry 
occurring where the conditions are different above and below 
but alike all around. Among the Gregarinida the spherical 
symmetry and symmetry passing from that into the radial, 
are such as appear to be congruous with the simple cir- 
cumstances of these creatures in the intestines of insects. 


170 MORPHOLOGICAL DEVELOPMENT 


But the relations of these lowest types to their environments 
are comparatively so indeterminate, and our knowledge of 


their actions so scanty, that little beyond negative evidence 
can be expected from the study of them. 

The like may be said of the Infusoria. These are more 
or less irregular. In some cases where the line of move- 
ment through the water is tolerably definite and constant 
we have a form that is approximately radial—externally at 
least. But usually, as shown in Figs. 137, 138, 139, there is 
either an unsymmetrical or an asymmetrical shape. And when 
one of these creatures is watched under the microscope, the 
congruity of this shape with the incidence of forces is mani- 
fest. For the movements are conspicuously varied and 
indeterminate—movements which do not expose any two 
or more sides of the mass to approximately equal sets of 
actions. 


§ 245. Among aggregates of the second order, as among 
aggregates of the first order,we find that of those possessing 
any definite shapes the lowest are spherical or spheroidal. 
Such are the Thalassicolle. These gelatinous bodies which 
float passively in the sea, and present in turn all their sides 
to the same influences, have their parts disposed with ap- 
proximate regularity all around a centre. In some orders 
of Foraminifera, as for instance the Nummulites, we have 
secondary aggregates the parts of which are spirally ar- 
ranged, approximately in harmony with the radial relations 
of the society to the environment; but we have other types 
in which the congregated units are distributed in ways not 
easily definable, and having to the environment relations that 
are obscure. Further, among these secondary aggregates in 
which the units, only physically integrated, have not had their 


THE GENERAL SHAPES OF ANIMALS. 171 


individualities merged into an individuality of a higher 
order, must be named the compound IJnfusoria. The 
cluster of Vorticelle in Fig. 144, will sufficiently exemplify 
them ; and the striking resemblance borne by its individuals 
to those of a radially-arranged cluster of flowers, will show 
how, under analogous conditions, the general principles of 
morphological differentiation are similarly illustrated in the 
two kingdoms. 


§ 246. Radial symmetry is usual in those aggregates of 
the second order that have their parts sufficiently differentiat- 
ed and integrated to give individualities to them as wholes. 
The Celenterata offer numerous examples of this. Solitary 
polypes—hydroid or helianthoid—mostly stationary, and 
when they move, moving with any side foremost, do not by 
locomotion subject their bodies to habitual contrasts of con- 
ditions. Seated with their mouths upwards or downwards, 
or else at all degrees of inclination, the individuals of a 
species taken together, are subject to no mechanical actions 
affecting some parts of their discs more than other parts. 
And this indeterminateness of attitude similarly prevents 
their relations to prey from being such as subject some of 
their prehensile organs to forces unlike those to which the 
rest are subject. The fixed end is differently conditioned 
from the free end, and the two are therefore different ; but 
around the axis running from the fixed to the free end the 
conditions are alike in all directions, and the form therefore 
is radial. Again, among many of the simple free- 
swimming Hydrozoa, the same general truth is exemplified 
under other circumstances. In acommon Medusa, advanc- 
ing through the water by the rhythmical contractions of 
its disc, the mechanical reactions are the same on all sides ; 
and as, from accidental causes, every part of the edge of the 
disc comes upwards in its turn, no part is permanently af- 
fected in a different way from the rest. Hence the radial 
form continues. 


172 MORPHOLOGICAL DEVELOPMENT 


In others of this same group, however, there occur forms 
which show us an incipient bilateralness; and help us to see 
how a more decided bilateralness may arise. Sundry of the 
Meduside are proliferous, giving origin to gemme from the 
body of the central polypite or from certain points on the 
edge of the disc; and this budding, unless it occurs equally 
on all sides, which it does not and is unlikely to do, must tend 
to destroy the balance of the disc, and to make its attitude 
less changeable. In other cases the growth of a large process 
from the edge of the disc on one side, as in Steenstrupia, Fig. 
257—a process that is perhaps the morphological equivalent 
of one of the gemmez just named—constitutes a similar modi- 
fication, and a cause of further modification. The existence 
of this process makes the animal no longer divisible into any 
two quite similar halves, except those formed by a plane 
passing through the process; and unless the process 1s 
exactly of the same specific gravity as the disc, it must tend 
towards either the lowest or the highest point, and must 
so serve to increase the bilateralness, by keeping the two 
sides of the disc similarly conditioned while the top and 
bottom are differently conditioned. Fig. 258 represents the 
underside of another Medusa, 
in which a more decided bi- 
lateralness is produced by the 
presence of two such process- 
es. Among the simple 
free-swimming Actinozoa, occur 
like deviations from radial sym- 
metry, along with like motions 

| through the water in bilateral 
attitudes. Of this a Cydippe is afamillar example. Though 
radial in some of its characters, as in the distribution of its 
meridional bands of locomotive paddles with their accompany- 
ing canals, this creature has a two-sided distribution of 
tentacles and various other parts, corresponding with its two- 
sided attitude in moving through the water. And in other 


THE GENERAL SHAPES OF ANIMALS. 173 


genera of this group, as in Cestum, Eurhamphea, and 
Callianira, that almost equal distribution of parts which 
characterizes the Beroe is quite lost. 

Here seems a fit place to meet the objection which some 
may feel to this and other such illustrations, that they amount 
very much to physical truisms. If the parts of a Medusa 
are disposed in radial symmetry around the axis of motion 
through the water, there will of course be no means of 
maintaining one part of its edge upwards more than another ; 
and the equality of conditions may be ascribed to the radiate- 
ness, as much as the radiateness to the equality of conditions. 
Conversely, when the parts are not radially arranged round 
the axis of motion, they must gravitate towards some one 
attitude, implying a balance on the two sides of a vertical 
plane—a bilateralness; and the two-sided conditions so 
necessitated, may be as much ascribed to the bilateralness as 
the bilateralness to the two-sided conditions. Doubt- 
less the form and the conditions are, in the way alleged, 
necessary correlates ; and in so far as it asserts this, the ob- 
jection harmonizes with the argument. To the difficulty 
which it at the same time raises by the implied question— 
Why make the form the result of the conditions, rather than 
the conditions the result of the form? the reply is this :— 
The radial type, both as being the least differentiated type 
and as being the most obviously related to lower types, must 
be taken as antecedent to the bilateral type. The indi- 
vidual variations which incidental circumstances produce in 
the radial type, will not cause divergence of a species from 
the radial type, unless such variations give advantages to the 
individuals displaying them; which there is no reason to sup- 
pose they will alwaysdo. Those occasional deviations from 
the radial type, which the law of the instability of the homo- 
geneous warrants us in expecting to take place, will, however, 
in some cases be beneficial ; and will then be likely to estab- 
lish themselves. Such deviations must tend to destroy the 
original indefiniteness and variability of attitude— must 


174 MORPHOLOGICAL DEVELOPMENT. 


cause gravitation towards an habitual attitude. And gravita- 
tion towards an habitual attitude having once commenced, 
will continually increase, where increase of it is not negatived 
by adverse agencies: each further degree of bilateralness 
rendering more decided the actions that conduce to bilateral- 
ness. If this reply be thought insufficient, it may be enforced 
by the further one, that as, among plants, the incident forces 
are the antecedents and the forms the consequents (changes of 
forces being in many cases visibly followed by changes of 
forms) we are warranted in concluding that the like order of 
cause and effect holds among animals. 


§ 247. Keeping to the same type but passing to a higher 
degree of composition, we meet more complex and varied 
illustrations of the same general laws. In the compound 
Celenterata, presenting clusters of individuals that are 
severally homologous with the solitary individuals last dealt 
with, we have to note both the shapes of the individuals thus 
united, and the shapes of the aggregates made up of them. 

Such of the fixed Hydrozoa and Actinozoa as form branched 
societies, continue radial ; both because their varied attitudes 
do not expose them to appreciable differences in their rela- 
tions to those surrounding actions which chiefly concern 
them (the actions of prey), and because such differences, even 
if they were appreciable, would be so averaged in their 
effects on the dissimilarly-placed members of each group as 
, to be neutralized in the race. Among the 
- vW, tree-like coral-polypedoms, as well as in 
such ramified assemblages of simpler poly- 

pes as are shown in Figs. 149, 150, we 
have, indeed, cases in many respects paral- 
lel to the cases of scattered flowers (§ 233), 
which though placed laterally remain radial, 
because no differentiating agency can act 
uniformly on all of them. Meanwhile, in the groups 
which these united individuals compose, we see the shapes of 


THE GENERAL SHAPES OF ANIMALS. 175 


plants further simulated under a further parallelism of con- 
ditions. The attached ends differ from the free ends as they 
do in plants; and the regular or irregular branches obvious- 
ly stand to environing actions in relations analogous to those 
in which the branches of plants stand. 

The members of those compound Oelenterata which move 
through the water by their own actions, in attitudes that are 
approximately constant, show usa more or less distinct two- 
sidedness. Diphyes, Fig. 259, furnishes an example. Each 


2bP 


of the largely-developed and modified polypites forming its 
swimming sacs is bilateral, in correspondence with the bi- 
lateralness of its conditions; and in each of the appended 
polypites the insertion of the solitary tentacle produces a 
kindred divergence from the primitive radial type. The 
aggregate, too, which here very much subordinates its mem- 
bers, exhibits the same conformity of structure to circum- 
stances. It admits of symmetrical bisection by a plane pass- 
ing through its two contractile sacs, or nectocalyces, but not 
by any other plane ; and the plane which thus symmetrically 
bisects it, is the vertical plane on the two sides of which its 
parts are similarly conditioned as it propels itself through 
the water. 

Another group of the oceanic Hydrozoa, the Physophoride, 
furnishes interesting evidence—not so much in respect of the 
forms of the united individuals, which we may pass over, as 
in respect of the forms of the aggregates. Some of these 
which are without swimming organs, have their parts sus- 
pended from air-vessels which habitually float on the surface 
of the water; and the distribution of their parts is asym- 


LG MORPHOLOGICAL DEVELOPMENT. 


metrical. The Physalia, Fig. 152, is an example. Here the 
relations of the integrated group of individuals to the environ- 
ment are indefinite ; and there is hence no agency tending 
to change that comparatively irregular mode of growth that 
is probably derived from a primordial 
an type of the branched Hydrozoa. 
VS So various are the modes of union 
a nK among the compound Ccelenterata, that 
it is out of the question to deal with 
them all. Even did space permit, it 
would be impracticable for any one but 
a professed naturalist, to trace through- 
out this group the relations between 
shapes and conditions of existence. The 
above must be taken simply as a few of 
the most significant and easily-interpret- 
able cases. 


152 § 248. In the sub-kingdom Mollus- 

coida, we meet with examples not wholly 

unlike the foregoing. Among the types assembled under 

this title there are simple individuals or aggregates of the 

second order, and societies or tertiary aggregates produced 

by their union. The relations of forms to forces have to be 
traced in both. 

Solitary Ascidians, fixed or floating, carry on an inactive 
and indefinite converse with the actions in the environment. 
Without power to move about vivaciously, and unable to 
catch any prey but that contained in the currents of water 
they absorb and expel, these creatures are not exposed to 
sets of forces that are equal on two or more sides ; and their 
shapes consequently remain vague. Though there are in 
them traces of symmetrical arrangement, probably due to 
their derivation, yet they are substantially asymmetrical. 
Fig. 156 is an example. Among the composite 
Ascidians, floating and fixed, the shape of the aggregate, 


THE GENERAL SHAPES OF ANIMALS. 17¢ 


partly determined by the habitual mode of gemmation and. 
partly by the surrounding conditions in each case, is in 
great measure indefinite. We can say no more about it than 
that it is not obviously at variance with the laws alleged. 

Evidence of a more positive kind occurs among those com- 
pound Molluscoida which are most like the compound 
Ccelenterata in their modes of union—the Polyzoa. Many of 
these form groups that are more or less irregular—spreading 
as films over solid surfaces, combining into sea-weed-like 
fronds, budding out from creeping stolons, or growing up 
into tree-shaped societies; and besides aggregating ir- 
regularly they are irregularly placed on surfaces inclined in 
all directions. Merely noting that this asymmetrical dis- 
tribution of the united individuals is explained by the 
absence of definiteness,in the relations of the aggregate to 
incident forces, it concerns us chiefly to observe that the 
united individuals severally exemplify the same truth as do 
similarly-united individuals among the Celenterata. While 
their internal organs, though said to have a trace of bi- 
lateralness, cannot be said to display any definite symmetry ; 
their external organs are completely radial. Averaging the 
members of each society, the ciliated tentacles they protrude 
are similarly related to prey on all sides; and therefore 
remain the same on all sides. This distribution of tentacles 
is not, however, without exception. Among the fresh-water 
Polyzoa there are some genera, as Plumatella and Crystatella, 
in which the arrangement of these parts is very decidedly 
bilateral. Some species of them show us such relations of 
the individuals to one another and to their surface of attach- 
ment, as give a clue to this modification ; but in other.species 
the meaning of this deviation from the radial type is not 
obvious. 


§ 249. In that somewhat heterogeneous assemblage of 
animals now classed, perhaps provisionally, as Annulotda, we 


begin again with simple aggregates of the second order, and 
os ore oO 12 


278 MORPHOLOGICAL DEVELOPMENT. 


ascend to aggregates in which we have seen reason to suspect 
a higher degree of composition. Good examples of the con- 
nexions between forms and forces occur ‘in this group. 
Among the lower annuloid types, the Planaria exemplifies 
the single bilateral symmetry which, even in very inferior 
forms, accompanies the habit of moving in one direction over 
a solid surface. Humbly organized as are these creatures 
and their allies the Nemertide, we see in them just as clearly 
as in the highest animals, that where the movements subject 
the body to different forces at its two ends, different forces 
on its under and upper surfaces, and like forces along its two 
sides, there arises a corresponding form, unlike at its extremi- 
ties, unlike above and below, but having its two sides alike. 
The Echinodermata furnish us with instructive illustrations 
—instructive because among types that are nearly allied, we 
meet with wide deviations of form answering to marked con- 
trasts in the relations to the environment. The facts fall 
into four groups. The Crinoidea, once so abundant 
and now so rare, present a radial symmetry answering to 
an incidence of forces that is equal on every side. In the 
general attitudes oftheir parts towards surrounding actions, 
they are like uniaxial plants or like polypes; and show, as 
they do, marked differences between the attached ends and 
the free ends, along with even distributions of parts all round 
their axes. In the Ophiuridea, proved to be near 
akin to the Crinoids, and in the Star-fishes, we have radial 
symmetry co-existing with very different habits ; but habits 
which nevertheless account for the maintenance of the form. 
Holding on to rocks and. weeds by its simple or branched 
arms, or by the suckers borne on the under surface of its 
rays, one of these creatures moves about not always with one 
side foremost, but with any side foremost. Consequently, 
averaging its, movements, its arms or rays are equally af- 
fected, and therefore remain the same on all sides. On 
watching?the sways of the common Sea-urchin, we are 
similarly furnished with an explanation of its spherical, or 


THE GENERAL SHAPES OF ANIMALS. 179 


rather its spheroidal, figure. Here the habit is not to move 
over any one approximately-flat surface; but the habit is to 
hold on by several surfaces on different sides at the same 
time. Frequenting crevices and the interstices among stones 
and weeds, the Sea-urchin protrudes the suckers arranged 
in ‘meridional bands over its shell, laying hold of objects now 
on this side and now on that, now above and now below: 
the result being, that it does not move.in all directions over 
one plane but in all directions through space. Hence the 
approach in general form towards spherical symmetry—an 
approach which is, however, restrained by the relations of 
the parts to the mouth and vent: the conditions not being 
exactly, the same at the two poles as at other parts of the 
surface. Still more significant is that deviation 
from this shape which occurs among such of the Echiznidea 
as have habitats of a different kind, and, consequently, dif- 
ferent habits. The genera Echinocyamus, Spatangus, Bris- 
sus, and Amphidotus, diverge markedly towards a bilateral 
structure. These creatures are found not on rocky shores 
but on flat sea-bottoms, and some of them only on oottoms 
of sand or mud. Here, there is none of that distribution of 
surfaces on all sides which makes the spheroidal form con- 
gruous with the conditions. Having to move about over an 
approximately-horizontal plane, any deviation of structure 
which leads to one side being kept always foremost, will be 
an advantage: greater fitness to function becoming possible 
in proportion as function, becomes fixed. Survival of the 
fittest will therefore tend to establish, under such conditions, 
a form that keeps the same part in advance—a form in 
which, consequently, the original radial symmetry diverges 
more and more towards bilateral symmetry. It may 
be well to add that the validity of these interpretations does 
not depend on the view taken of the alliances of the Echino- 
derms, and their primitive type of symmetry. If, as Pro- 
fessor Huxley contends, the Echinoderms, having bilaterai 


larvee, cannot be held akin to those lower types in which the 
12 @ 


180 MORPHOLOGICAL DEVELOPMENT 


radial structure is constant and complete; it does not follow 
that the above reasonings are erroneous. On the contrary 
the derivation of these radially-symmetrical forms from forms 
not radially-symmetrical, would show how entirely the 
structure of the organism is moulded by the distribution of 
forces to which its mode of life exposes it. 

The remaining Annuloida, most of them parasitic, must 
be passed over. . Living within the bodies of other creatures, 
they have their forms determined by conditions that are too 
obscure to be satisfactorily dealt with. 


§ 250. Very definite and comparatively uniform, are the 
relations between shapes and -circumstances among the 
Annulosa—including under that title the Annehda and the 
Articulata. The agreements and the: disagreements are 
equally instructive. 

At one time or other of its life, if not throughout its life, 
every annulose animal is locomotive; and its temporary 
or permanent locomotion, being carried on with one end 
habitually foremost and one surface habitually uppermost, 
it fulfils those conditions under which bilateral symmetry 
arises. Accordingly, bilateral symmetry is traceable through- 
out the whole of this sub-kingdom. Traceable, we must 
say, because,’though it is extremely conspicuous in the 
immense majority of annulose types, it is to a consider- 
able extent obscured where obscuration is to be expected. 
The embryos of the Tubicole, after swimming about 
awhile, settle down and build themselves tubes, from which 
they protrude their heads; and in them, or in some of 
them, the bilateral symmetry is disguised by the develop- 
ment of head-appendages in an all-sided manner. The 
tentacles of Teredella are distributed much in the same way 
as those of a polype. The breathing organs in Sabella 
unispira, Fig. 260, do not correspond on opposite sides of a 
median plane. Even here, however, the body retains its 
primitive bilateralness; and it is further to be remarked that 


THE GENERAL SHAPES OF ANIMALS. 181 


this loss of bilateralness in the external appendages, does not 
occur where the relations to external conditions continue 
bilateral: witness the Serpula, Fig. 261, which has its: 


respiratory tufts arranged in a two-sided way, under the 
two-sided conditions involved by the habitual position of 
its tube. 

The community of symmetry among the higher Annulosa, 
has an unobserved significance. That Flies, Beetles, Lob- 
sters, Centipedes, Spiders, Mites, have in common the 
characters, that the end which moves in advance differs from 
the hinder end, that the upper surface differs from the under 
surface, and that the two sides are alike, is a truth received.as 
a matter of course. After all that has been said above, how- 
ever, it will be seen to have a meaning not to be overlooked ; 
since it supplies a million-fold illustration of the laws that 
have been set forth. It is needless to give diagrams. Every 
reader can call to mind the unity indicated. 

While, however, annulose animals repeat so uniformly 
these traits of structure, there are certain other traits in 
which they are variously contrasted; and their contrasts 
have to be here noted, as serving further to build up the 
general argument. In them we see the stages through 
which bilateral symmetry becomes gradually more marked, as . 
the conditions it responds to become more decided. 


182 MORPHOLOGICAL DEVELOPMENT. 


common Earth-worm may be instanced as a member of 
this sub-kingdom that is among the least-conspicuously 
bilateral. Though internally its parts have a two-sided 
arrangement; and though the positions of its orifices give it 
an external two-sidedness, at the same time that they estab- 
lish a difference between the two ends; yet its two-sidedness 
is not strongly marked. The form deviates but little from 
what we have distinguished as triple bilateral symmetry: if 
the creature is cut across the middle, the head and tail ends 
are very much alike; if cut in two along its axis by a hori- 
zontal plane, the under and upper halves are very much 
alike ; and if cut in two along its axis by a vertical plane, 
the two sides are quite alike. Figs. 263 and 264 will make 
this clear. Such creatures as the Julus and the 
Centipede, may be taken as showing a transition to double 
bilateral symmetry. Besides being divisible into exactly 
sumilar halves by a vertical plane passing through the axis, 
one of these animals may be bisected transversely into parts 
that differ only slightly; but if cvt in two by a horizontal 
plane passing through the axis, the under and upper halves 
are decidedly unlike. Figs. 265, 266, exhibit these 
traits. Among the isopodous crustaceans, the departure 
from these low types of symmetry is more marked. As 


2 sot 
~<2226S 0S ESESESEEEEE ICDS HE 


GF 
265 


266) 
rttt THHNTALATY 
BRED, 268 
ZIPP cs 
269 . 
J 
WAN 


shown in Figs. 267 and 268, the contrast between the upper 
and under parts is greater, and the head and tail ends differ 


THE GENERAL SHAPES OF ANIMALS. 183 


more obviously. In all the higher Articulata, the 
unlikeness between the front half and the hind half has 
become ‘conspicuous: there is in them single bilateral 
symmetry of so pronounced a kind, that no other resem- 
blance is suggested than that between the two sides. By 
Figs. 269 and 270, representing a decapodous crustacean 
divided longitudinally and transversely, this truth is made 
manifest. On calling to mind the habits of the 
creatures here drawn and described, it will be seen that 
they explain these forms. The incidence of forces is the 
same all around the Karth-worm as it burrows through the 
compact ground. The Centipede, creeping amid loose soil or 
débris or beneath stones, insinuates itself between solid sur- 
faces—the interstices being mostly greater in one dimension 
than in others. And all the higher Annulosa, moving about 
as they do over exposed objects, have their dorsal and 
ventral parts as dissimilarly acted upon as are their two ends. 
One other fact only respecting annulose animals needs to 
be noticed under this head—the fact, namely, that they 
become unsymmetrical where their parts.are unsymmetrically 
related to the environment. The common Hermit-crab 
serves as an instance. Here, in addition to the unlikeness of 
the two sides implied by that curvature of the body which fits 
the creature to the shell it inhabits, there is an unlikeness 
due to the greater development of the limbs, and especially 
the claws, on the outer side. As in the embryo of the 
Hermit-crab the two sides are alike; and as the EEA may 
be taken to represent the type from which the 
Hermit-crab has been derived; we have in 
this case evidence that a symmetrically-bi- 
Jateral form has been moulded into an unsym- 
metrically-bilateral form, by the action of un- 
symmetrically-bilateral conditions. A further . 
illustration is supplied by Bopyrus, Fig. 271i: SUL 
a parasite the habits of which similarly account for its’ dis 
torted shape. 


184 MORPHOLOGICAL DEVELOPMENT. 


- § 251. Among the Mollusca we find more varied relations 
between shapes and circumstances. -Some of them are 
highly instructive. 

Mollusks of one order, the Pteropoda, swim in the sea 
much in the same way that butterflies fly in 
the air, and have shapes not altogether unlike 
those of butterflies. Fig. 272 represents one 
of these creatures. That its bilaterally-sym- 
metrical shape harmonizes with itsbilaterally- 
symmetrical conditions is sufficiently obvious. 

Among the Lamellibranchiata, we have 
: diverse forms accompanying diverse modes of 
life. Such of them as frequently move about, like the fresh- 
water Mussel, have their two valves and the contained parts 
alike on the opposite sides of a vertical plane: they are 
bilaterally symmetrical in conformity with their mode of 
movement. The marine Mussel, too, though habitually 
fixed, and though not usually so fixed that its two valves are 
similarly conditioned, still retains that bilateral symmetry 
which is characteristic of the order; and it does this because 
in the species considered as a whole, the two valves are not 
dissimilarly conditioned. If the positions of the various 
individuals are averaged, it will be seen that the differenti- 
ating actions neutralize one another. In certain 
other fixed Lamellibranchs, however, there is a considerable 
deviation from bilateral symmetry}; and it is a deviation of - 
the kind to be-anticipated under the circumstances. Where 
one valve is always downwards, or next to the surface of 
attachment, while the other valve is always upwards, or next 
to the environing water, we may expect to find the two 
valves become unlike. This we do find: witness the Oyster. 
In the Oyster, too, we see a further irregularity. There isa 
great indefiniteness of outline, both in the shell and in the 
animal — an indefiniteness made manifest by comparing 
different individuals. We have but to remember that growing 
clustered together, as Oysters do, they must interfere with 


THE GENERAL SHAPES OF ANIMALS. 185 


one another in various ways and degrees, to see how the 
indeterminateness of form and the variety of form are 
accounted for. 

Among the Gasteropods, modifications of a more definite 
kind occur. ‘In all Mollusks,” says Professor Huxley, 
“‘the axis of the body is at first straight, and its parts are 
arranged symmetrically with regard to a longitudinal verti- 
cal plane, just as in a vertebrate or an articulate embryo.” 
In some Gasteropods, as the Chiton, this bilateral sym- 
metry is retained—the relations of the body to surround- 
ing actions not being such as to disturb it. But in those 
more numerous types that have spiral shells, there is a 
marked deviation from bilateral symmetry, as might be ex- 
pected. “This asymmetrical over-development never affects 
the head or foot of the mollusk:” only those parts which, 
by inclosure in a shell, are protected from environing actions, 
lose their bilateralness; while the external parts, subjected by 
the movements of the creatures to bilateral conditions, remain 
bilateral. Here, however, a difficulty meets us. Why is it 
that the naked Gasteropods, such as our common slugs, 
deviate from bilateral symmetry, though their modes of 
movement are those along with which complete bilateral 
symmetry usually occurs? ‘The reply is, that their devia- 
tions from bilateral symmetry are probably inherited, and 
that they are maintained in such parts of their organiza- 
tion as are not exposed to bilaterally-symmetrical conditions. 
There is reason to believe that the naked Gasteropods are 
descended from Gasteropods that had shells: the evidence 
being that the naked Gasteropods have shells during the 
early stages of their development, and that some of them 
retain rudimentary shells throughout life. Now the shelled 
Gasteropods deviate from bilateral symmetry in the dis- 
position of both the alimentary system and the reproductive 
system. The naked Gasteropods, in losing their shells, have 
lost that. immense one-sided development of the alimentary 
system which fitted them to their shells, and have acquired 


186 MORPHOLOGICAL DEVELOPMENT 


that bilateral symmetry of external figure which fits them 
for their habits of locomotion; but the reproductive system 
remains one-sided, because, in respect to it, the relations to 
external conditions remain one-sided. 

The Cephalopods, which are interpretable as higher de- 
velopments on the Gasteropod type, show us bilaterally-sym- 
metricalexternal forms along with habits of movement through 
the water in two-sided attitudes. At the same time, in the radial 
distribution of the arms, enabling one of these creatures to 
' take an all-sided grasp of its prey, we see how readily upon one 

kind of symmetry there may be partially developed another 
kind of symmetry, where the relations to conditions favour it. 

§ 252. The Vertebrata illustrate afresh the truths which 
we have already traced among the Annulosa. Flymg 
through the air, swimming through the water, and running 
over the earth as vertebrate animals do, in common with 
annulose animals, they are, in common with annulose ani- 
mals, different at their anterior and posterior ends, different 
at their dorsal and ventral surfaces, but alike along their 
two sides. This single bilateral symmetry remains constant 
under the extremest modifications of form. Among fish 
we see it alike in the horizontally-flattened Skate, in the 
vertically-flattened Bream, in the almost spherical Diodon, 
and in the greatly-elongated Syngnathus. Among reptiles 
the Turtle, the Snake, and the Crocodile all display it. And 
under the countless modifications of structure displayed by 
birds and mammals, it remains conspicuous. 

A less obvious fact which it concerns us to note among the 
Vertebrata, parallel to one which we noted among the 
Annulosa, is that whereas the lower vertebrate forms deviate 
but little from triple bilateral symmetry, the deviation be- 
comes great as we ascend. Figs. 273 and 274 show how, 
besides being divisible into similar halves by a vertical plane 
passing through its axis, a Fish is divisible into halves that 
are not very dissimilar by a horizontal plane passing through 


THE GENERAL SHAPES OF ANIMALS. 187 


.ts axis, and also into other not very dissimilar halves by a 
plane cutting it transversely. If, as shown in Figs. 275 
and 276, analogous sections be made of a superior Reptile, the 
divided parts differ more decidedly. When a Mammal anda 
Bird are treated in the same way, as shown in Figs. 277, 
278, and Figs. 279, 280, the parts marked off by the divid- 


273 Ae 
an 
“~~ if {e) y 
a K 
af. 246 


ea 


ing planes are unlike in far greater degrees. On considering 
the mechanical converse between organisms of these several 
types and their environments—on remembering that the 
fish habitually moves through a homogeneous medium of 
nearly the same specific gravity as itself, that the terrestrial 
reptile either crawls on the surface or raises itself very in- 
completely above it, that the more active mammal, having 


a 


\ 


188 MORPHOLOGICAL DEVEIOPMENT. 


its supporting parts more fully developed, thereby has the 
under half of its body made more different from the upper 
half, and that the bird is subject by its mode of life to yet 
another set of actions and reactions; we shall see that these 
facts are quite congruous with the general doctrine, and fur- 
nish further support to it. 

One other significant piece of evidence must be named. 
Among the Annulosa we found unsymmetrical bilateralness 
in creatures having habits exposing them to unlike conditions 
on their two sides; and among the Vertebrata we find parallel 
cases. They are presented by the Pleuronectide—the order 
of distorted flat fishes to which the Sole and the Flounder 
belong. On the hypothesis of evolution, we must conclude 
that fishes of this order have arisen from an ordinary bila- 
terally-symmetrical type of fish, which, feeding at the 
bottom of the sea, gained some advantage by placing itself 
with one of its sides downwards, instead of maintaining the 
vertical attitude. Besides the general reason there are speci+ 
fic reasons for concluding this. In the first place, the young 
Sole or Flounder is bilaterally symmetrical—has its eyes on 
opposite sides of its head, and swims in the usual way. In the 
second place, the metamorphosis which produces the unsym- 
metrical structure sometimes does not take place—there are 
abnormal Flounders that swim vertically, like other fishes. 
In the third place, the transition from the symmetrical 
structure to the unsymmetrical structure may be traced. 
Almost incredible though it seems, one of the eyes is 
transferred from the under-side of the head to the upper- 
side. Until lately it was supposed that the change by 
which the two eyes, originally placed on opposite sides, come 
‘to be placed on the same side, is effected by a distortion 
of the cranium ; but it is now asserted that actual migration 
of an eye occurs. According to Prof. Steenstrup, the eye 
‘passes. between the ununited bones of the skull ; but according 
to Prof. Thomson, it passes under the skin. Be the course of 
-the metamorphosis what it may, however, it furnishes several 


THE GENERAL SHAPES OF ANIMALS. 18¢ 


remarkable illustrations of the way in which forms become 
moulded into harmony with incident forces. For besides 
this divergence front bilateral symmetry involved by the 
presence of both eyes upon the upper side, there is a further 
divergence from bilateral symmetry involved by the differ- 
entiation of the two sides in respect to the contours of their 
surfaces and the sizes of their fins. And then, what is still 
more significant, there is a near approach to likeness be- 
tween the halves that were originally unlike, but are, unde 
the new circumstances, exposed to like conditions. Ths 
body is divisible into similarly-shaped parts by a plane 
cutting it along the side from head to tail: ‘“ the dorsal and 
ventral instead of the lateral halves become symmetrical in 
outline and are equipoised.” 


§ 253. Thus, little as there seems in common between the 
shapes of plants and the shapes of animals, we yet find, on 
analysis, that the same general truths are displayed by 
both. The one ultimate principle that in any organism 
equal amounts of growth take place in those directions in 
which the incident forces are equal, serves as a key to the 
phenomena of morphological differentiation. By it we are 
furnished with interpretations of those likenesses and un- 
likenesses of parts, which are exhibited in the several kinds 
of symmetry; and when we take into account inherited 
effects, wrought under ancestral conditions contrasted in 
various ways with present conditions, we are enabled to 
comprehend, in a general way, the actions by which animals 
have been moulded into the shapes they possess. 

To fill up the outline of the argument, so as to make it 
correspond throughout with the argument respecting vegetal 
forms, it would be proper here to devote a chapter to the 
differentiations of those homologous segments out of which 
animals of certain types are composed. Though, among most 
animals of the third degree of composition, such as the root- 
ed Hydrozoa, the Polyzoa, and the Ascidioida, the united 


190 MORPHOLOGICAL DEVELOPMENT. 


individuals are not reduced to the condition of segments of a 
composite individual, and do not display any marked differ- 
entiations; yet there are some animals in which such 
subordinations, and consequent heterogeneities, occur. The 
oceanic Hydrozoa form one group; and we have seen 
reason to conclude that the Annulosa form another group. 
It is not worth while, however, to occupy space in detailing 
these unlikenesses of homologous segments, and seeking 
specific explanations of them. Among the oceanic Hydrozoa 
they are extremely varied ; and the habits and derivations of 
these creatures are so little known, that there are no adequate 
data for interpreting the forms of the parts in terms of their 
relations to the environment. Conversely, among the An- ‘ 
nulosa those differentiations of the homologous segments 
which accompany their progressing integration, have so 
much in common, and have general causes which are so ob- 
vious, that it is needless to deal with them at any length. 
They are all explicable as due to the exposure of different parts 
of the chain of segments to different sets of actions and re- 
actions: the most general contrast being that between the 
anterior segments and the posterior segments, answering to. 
the most general contrast of conditions to which annulose 
animals subject their segments; and the more special con- 
trasts answering to the contrasts of conditions entailed by 
their more special habits. 

Were an exhaustive treatment of the subject practicable, 
there should here, also, come a chapter devoted to the internal 
structures of animals—meaning, more especially, the shapes 
and arrangements of the viscera. The relations between 
forms and forces among these inclosed parts, are, however, 
mostly too obscure to allow of interpretation. Protected as 
the viscera are in great measure from the incidence of ex- 
ternal forces, we are not likely to find much correspondence 
between their distribution and the distribution of external 
forces. In this case the influences, partly mechanical, partly 
physiological, which the organs exercise on one another,, 


THE GENERAL SHAPES OF ANIMALS. 191 


become the chief causes of their changes of figure and.ar- 
rangement ; and these influences are complex and indefinite. 
One general fact may, indeed, be noted—the fact, namely, 
that the divergence towards asymmetry which generally 
characterizes the viscera, is marked among those of them 
which are’ most removed from mechanical converse with the 
environment, but not so marked among those of them which 
are less removed from such converse. Thus while, through- 
out the Vertebrata, the alimentary system, with the exception 
of its two extremities, is asymmetrically arranged, the re- 
spiratory system, which occupies one end of the body, ge- 
nerally deviates but little.from bilateral symmetry, and the 
reproductive system, partly occupying the other end of the 
body, is in the main bilaterally symmetrical: such deviation 
from bilateral symmetry as occurs, being found in its most 
interiorly-placed parts, the ovaries. Just indicating these 
facts as having a certain significance, it will be best to leave 
this part of the subject as too involved for detailed treat- 
ment. 

Internal structures of one class, however, not included 
among the viscera, admit of general interpretation—struc- 
tures jwhich, though ‘internal, are brought into tolerably- 
direct relations with the environing forces, and are therefore 
subordinate in their forms to the distribution of those forces. 
These: internal structures it will be desirable to deal with 
at some length ; both because they furnish important illustra- 
tions?enforcing the general argument, and because an inter- 
pretation of them which we have ‘seen reason to reject, 
cannot be rejected without raising the demand for some other 
interpretation. 


CHAPTER XV. 
THE SHAPES OF VERTEBRATE SKELETONS. 


§ 254. Wuen an elongated mass of any substance is ‘ 
transversely strained, different parts of the mass are ex- 
posed to forces of opposite kinds. If, for example, a bar 
of metal or wood is supported at its two ends, as shown in 
Fig. 281, and has to bear a weight on its centre, its lower 


—— _— 
sx: — 
So as'en - —_—ss 
-—-.~. 
-——- E _—_-—--—— 


/\ 


part is thrown into a state of tension, while its upper part is 
thrown into a state of compression. As will be manifest to 
any one who has observed what happens on breaking a stick 
across his knee, the greatest degree of tension falls on the 
fibres that form the convex surface, while the fibres forming 
the concave surface are subject to the greatest degree of 
compression. Between these extremes the fibres at different 
depths are subject to different forces. Progressing upwards 
from the under surface of the bar shown in Fig. 281, the 
tension of the fibres becomes less; and progressing down- 
wards from the upper surface, the compression of the fibres 
becomes less; until, at a certain distance between the twa 
surfaces, there is a place at which the fibres are neither ex- 
tended nor compressed. This, shown by the dotted line in 


YHE SHAPES OF VERTEBRATE SKELETONS. 193 


the figure, is called in mechanical language the “ neutral 
axis.” It varies in position with the nature of the substance 
strained: being, in common pine-wood, at a distance of about 
five eighths of the depth from the upper surface or three 
eighths from the under surface. Clearly, if such a piece of 
wood instead of being subject to a downward force is secured 
at its ends and subject to an upward force, the distribution 
of the compressions and tensions will be reversed, and the 
neutral axis will be nearest to the upper surface. Fig. 282 
represents these opposite attitudes of the bar and the changed 


position of its neutral axis: the arrow indicating the direc- 
tion of the force producing the upward bend, and the faint 
dotted line a, showing the previous position of the neutral axis. 
Between the two neutral axes will be seen a central space. 
and it is obvious that when the bar has its strain from time 
to time reversed, the repeated changes of its molecular con- 
dition must affect the central space in a way different from 
that in which they affect the two outer spaces. Fig. 283 is 
a diagram conveying some idea of these contrasts in molecular 
condition. If A BC D be the middle part of a bar thus 
treated, while G H and K L are the alternating neutral 
axes; then the forces to which the bar is in each case subject, 
may be readily shown. Supposing the deflecting force to 
be acting in the direction of the arrow H, then the tensions 
to which the fibres between G and F are exposed, will be 
represented by a series of lines increasing in length as the 
distance from G increases; so that the triangle G F M, will 
express the amount and distribution of all the molecular 
tensions. But the molecular compressions throughout the 
space from G to E, must balance the molecular tensions; 
and hence, if the triangle G E N be made equal to the tri- 


VOL, II. 13 


194 MORPHOLOGICAL DEVELOPMENT. 


angle G F M, the parallel lines of which it is composed (here 
dotted for the sake of distinction) will express the amount 


and distribution of the compressions between E and G. 
Similarly, when the deflecting force is in the direction of the 
arrow fF’, the compressions and tensions will be quantitatively 
symbolized by the triangle K F O, and K EP. And 
thus the several spaces occupied by full lines and by dotted 
lines and by the two together, will represent the different 
actions to which different parts of the transverse section are 
subject by alternating transverse strains. Here then it is 
made manifest to the eye that the central space between @ 
and K, is differently conditioned from the spaces above and 
below it; and that the difference of condition is sharply 
marked off. The fibres forming the outer surface C D, are 
subject to violent tensions and violent compressions. Pro- 
gressing inwards the tensions and compressions decrease— 
the tensions the more rapidly. As we approach the point G, 
the tensions to which the fibres are alternately subject, bear 
smaller and smaller ratios to the compressions, and disappear 
at the point G. Thence-to the centre occur compressions 


THE SHAPES OF VERTEBRATE SKELETONS. 195 


only, of alternating intensities, becoming at the centre small 
and equal; and from the centre we advance, through a 
reverse series of changes, to the other side. 

Thus it is demonstrable that any substance in which the 
power of resisting compression is unequal to the power of 
resisting tension, cannot be subject to alternating transverse 
strains, without having a central portion differentiated in its 
conditions from the outer portions, and consequently dif- 
ferentiated in its structure. This conclusion may easily be 
verified by experiment. Ifsomething having a certain tough- 
ness but not difficult to break, as a thick piece of sheet lead, 
be bent from side to side till it is broken, the surface of frac- 
ture will exhibit an unlikeness of texture between the inner 
and outer parts. 


§ 255. And now for the application of this seemingly-irre- 
levant truth. Though it has no obvious connection with the 
interpretation of vertebral structure, we shall soon see that it 
fundamentally concerns us. 

The simplest type of vertebrate animal, the fish, has a 
mode of locomotion which involves alternating transverse 
strains. It is not, indeed, subjected to alternating transverse 
strains by some outer agency, as in the case we have been 
investigating: it subjects itself to them. But though the 
strains are here internally produced instead of externally 
produced, the case is not therefore removed into a wholly 
different category. For sup- | 
posing Fig. 284 to represent (<-> 
the outline of a fish when ee 
bent on one side (the dotted lines representing its outline 
when the bend is reversed), it is clear that part of the sub- 
stance forming the convex half must be in a state of tension. 
This state of tension implies the existence in the other half 
of some counter-balancing compression. And between the 
two there must be a neutral axis. The way in which this 


conclusion is reconcilable with the fact that there is tension 
18 * 


= 


196 MORPHOLOGICAL DEVELOPMENT. 


somewhere in the concave side of a fish, since the curve is 
caused by muscular contractions on the concave side, will be 
made clear by the rude illustration which a bow supplies. 
A bow may be bent by a thrust against its middle (the two 
ends being held back), or it may be bent by contracting 
a string that unites its ends; but the distributions of me- 
chanical forces within the wood of the bow, though not quite 
alike in the two cases, will be very similar. Now while the 
muscular action on the concave side of a fish differs from that 
represented by the tightened string of a bow, the difference 
is not such as to destroy the applicability of the illustration : 
the parallel, holds so far as this, that within that portion of 
the fish’s body which is passively bent by the contracting 
muscles, there must be, as in a strung bow, a part in com- 
pression, a part in tension, and an intermediate part which 
is neutral. 

Recognizing the fact that even in the developed fish with 
its complex locomotive apparatus, this law of the transverse 
strain holds in a qualified way, we shall understand how 
much more it must hold in any form that may be supposed 
to initiate the vertebrate type—a form devoid of that seg- 
mentation by which the vertebrate type is more or less cha- 
racterized. We shall see that assuming a rudimentary 
animal still simpler than the Amphiorus, to have a feeble 
power of moving itself through the water by the undulations 
of its body, or some part of its body, there will necessarily 
come into play certain reactions that must affect the median 
portion of the undulating mass in a way unlike that in 
which they affect its lateral portions. And if there exists in 
this median portion a tissue that keeps its place with any 
constancy, we may expect that the differential conditions 
produced in it by the transverse strain, will initiate a dif- 
ferentiation. It is true that the distribution of the viscera 
in the Amphiorus, Fig. 191, and in the type from which 
‘we ay suppose it to arise, 1s such-as to interfere with this 


THE SHAPES OF VERTEBRATE SKELETONS. 197 


process. Itis also true that the actions and reactions de- 
scribed would not of themselves give to the median portion 


a cylindrical shape, like that of the cartilaginous rod run- 
ning along the back of the Amphiozus. But what we have 
here to note in the first place is, that these habitual alternate 
flexions have a tendency to mark off from the outer parts an 
unlike inner part, which may be seized hold of, main- 
tained, and further modified, by natural selection, should 
any advantage thereby result. And we have to note in the 
second place, that an advantage is likely to result. The con- 
tractions cannot be effective in producing undulations, un- 
less the general shape of the body is maintained. External 
muscular fibres unopposed by an internal resistent mass, 
would cause collapse of the body. To meet the require- 
ments there must be a means of maintaining longitudinal 
rigidity without preventing bends from side to side; and such 
a means is presented by a structure initiated as described. 
In brief, whether we have or have not the actual cause, we 
have here at any rate “a true cause.” Though there are 
difficulties in tracing out the process in a specific way, it may 
at least be said that the mechanical genesis of this rudiment- 
ary vertebrate axis is quite conceivable. And even the 
difficulties may, I think, be much more fully met than 
would at first sight seem possible. 

What is to be said of the other leading trait which the 
simplest vertebrate animal has in common with all higher 
vertebrate animals—the segmentation of its lateral mus- 


198 - MORPHOLOGICAL DEVELOPMENT. 


cular masses? Is this, too, explicable on the mechanical | 
hypothesis? Have we, in the perpetual transverse strains, | 
a cause for the fact that while the rudimentary vertebrate 
axis is without any divisions, there are definite divisions 
of the substance forming the animal’s sides? I think we 
have. A glance at the distribution of forces under the 
transverse strain, as represented in the foregoing diagrams, 
will show how much more severe is the strain on the outer 
parts than on the inner parts; and how, consequently, any 
modifications of structure eventually necessitated, will arise 
peripherally before they arise centrally. The perception of ' 
this may be enforced by a simple experiment. Take a stick 
of sealing-wax and warm it slowly and moderately before 
the fire, so as to give it a little flexibility. Then bend it 
gently until it is curved into a semi-circle. On the convex 
surface small cracks will be seen, and on the concave sur- 
face wrinkles; while between the two the substance remains 
undistorted. If the bend be reversed and re-reversed, time 
after time, these cracks and wrinkles will become fissures 
which gradually deepen. But now, if changes of this class, en- 
tailed by perpetual transverse strains, commence superficially, 
as they manifestly must; there arise the further questions—~ 
What will be the special modifications produced under these 
special conditions? and through what stages will these modifi- 
cations progress ? Every one has literally at hand an example’ 
of the way in which a flexible external layer that is now ex- 
tended and now compressed, by the bending of the mass it 
covers, becomes creased ; and a glance at the palms and the 
fingers will show that the creases are near one another’ 
where the skin is thin, and far apart where the skin is thick. ° 
Between this familiar case and the case of the rhinoceros- 
hide, in which there are but a few large folds, various grada- 
tions may be traced. Now the like must happen with the 
increasing layers of contractile fibres forming the sides of' 
the muscular tunic in such a type as that supposed. The 
bendings will produce in them small wrinkles while they are 


THE SHAPES OF VERTEBRATE SKELETONS. 199 


thin, but more decided and comparatively distant fissures as 
they become thick. Fig 289, which is a 
horizontal longitudinal section, shows 
how these thickening layers will adjust 
themselves on the convex and the con- 
cave surfaces, supposing the fibres of exe Ler 
which they are composed. to be oblique, 

as their function requires; and it is not difficult to see that 
when once definite divisions have been established, they will 
advance inwards as the layers develop ; and will so produce 
a series of muscular bundles. Here then we have something 
like the myocommata which are traceable in the Amphiorus, 
and are conspicuous in all superior fishes. 


489 


§ 256. These speculative conceptions I have ventured to 

present with the view of showing that the hypothesis of the 
mechanical genesis of vertebrate structure, is not wholly at 
fault when applied to the most rudimentary vertebrate ani- 
mal. Lest it should be alleged that the question is begged 
if we set out with a type which, like the Amphiovus, already 
displays segmentation throughout its muscular system, it 
seemed needful to indicate conceivable modes in which there 
may have been mechanically produced those leading traits 
that distinguish the Amphiorus. It seemed needful to 
assign an origin for the notochord; and. to this we see a 
clue in the differentiating effects of the transverse strain. It 
seemed needful to account for the existence of muscular 
divisions while yet there are no vertebral divisions; and for 
this, also, the transverse strain furnishes a feasible reason. 

But now, having shown that the actions and reactions in- 
volved by its mode of locomotion, are possible causes of those 
rudimentary structures which the simplest vertebrate animal 
presents, let us return to the region of established fact, and 
consider whether such actions and reactions as we actually 
witness, are adequate causes of those observed differentiations 
and integrations which distinguish the more-developed ver- 


200 MORPHOLOGICAL DEVELOPMENT. | 


tebrate animals. Let us see whether the theory of mechani- 
cal genesis afford us a deductive interpretation of the: in- 
ductive generalizations. 

Before proceeding, we must note a process of functional 
adaptation which here co-operates with natural selection. IL 
refer to the habitual formation of denser tissues at those 
parts of an organism which are exposed to the greatest 
strains—either compressions or tensions. Instances of hard- 
ening under compression are made familiar to us by the 
skin. We have the general contrast between the soft skin 
covering the body at large, and the indurated skin covering 
the inner surfaces of the hands and the soles of the feet. 
We have the fact that even within these areas the parts on 
which the pressure 1s habitually greatest, have the skin 
habitually thickest; and that in each person special points 
exposed to special pressures become specially dense—often 
as dense as horn. Further, we have the converse fact, that 
the skin of little-used hands becomes abnormally thin—even 
losing, in places, that ribbed structure which distinguishes 
skin subject to rough usage. Of increased density directly 
ollowing increased tension, the skeletons, whether of men 
or animals, furnish abundant evidence. Anatomists easily 
discriminate between the bones of a strong man and those of 
a weak man, by the greater development of those ridges and 
crests to which the muscles are attached ; and naturalists, on 
comparing the remains of domesticated animals with those 
of wild animals of the same species, find kindred differences. 
The first of these facts shows unmistakably the immediate 
effect of function on structure, and by obvious alliance with 
it the second may be held to do the same—both implying 
that the deposit of dense substance capable of great resist- 
ance, habitually takes place at points where the tension is 
_ excessive. . 

Taking into account, then, this adaptive process, con- 
tinually aided by the survival of individuals in which it 
has taken place most rapidly, we may expect, on tracing up 


THE SHAPES OF VERTEBRATE SKELETONS. 201 


the evolution of the vertebrate axis, to find that as the mus- 
cular power becomes greater there arise larger and harder 
masses of tissue, serving the muscles as points d’appui ; and 
that these arise first in those places where the strains are 
greatest. Now this is just what we do find. The myocom- 
mata are so placed that their actions are likely to affect first 
that upper coat of the notochord, where there are found 
“‘quadrate masses of somewhat denser tissue,’ which ‘“ seem 
faintly to represent neural spines,” even in the Amphiowus. 
It is by the development of the neural spines, and after them 
of the hemal spines, that the segments of the vertebral 
column are first marked out; and under the increasing strain 
of more-developed myocommata, it is just these peripheral 
appendages of the vertebral segments that must be most 
subject to the forces which cause the formation of denser 
tissue. It follows from the mechanical hypothesis that as 
the muscular segmentation must begin externally and pro- 
gress inwards, so, too, must the vertebral segmentation. 
Besides thus finding reason for the fact that in fishes with 
wholly cartilaginous skeletons, the vertebral segments are 
indicated by these processes, while yet the notochord is un- 
segmented; we find a like reason for the fact that the tran- 
sition from the less-dense cartilaginous skeleton to the more- 
dense osseous skeleton, pursues a parallel course. In the 
existing Lepidosiren, which by uniting certain piscine and 
amphibian characters betrays its close alliance with primitive 
types, the axial part of the vertebral column is unossified, 
while there is ossification of the peripheral parts. Similarly 
with numerous genera of fishes classed as paleozoic. The 
fossil remains of them show that while the neural and hemal 
spines consisted of bone, the central parts of the vertebra 
were not bony. It may in some cases be noted, too, both in 
extant and in fossil forms, that while the ossification is com- 
plete at the outer extremities of the spines it is incomplete 
at their inner extremities—thus similarly implying centri- 
petal development. 


202 MORPHOLOGICAL DEVELOPMENT. 


§ 257. After these explanations the process of eventual 
segmentation in the spinal axis itself, will be readily under- 
stood. The original cartilaginous rod has to maintain longi- 
tudinal rigidity while permitting lateral flexion. As fast as- 
it becomes definitely marked out, it will begin to concentrate 
within itself a great part of those pressures and tensions 
caused by transverse strains. As already said, it must be 
acted upon much in the same manner as a bow, though it is 
bent by forces acting in a more indirect way ; and like a bow, . 
it must, at each bend, have the substance of its convex side 
extended and the substance of its concave side compressed. 
So long as the vertebrate animal is small or inert, such a 
cartilaginous rod may have sufficient strength to withstand 
the muscular strains ; but, other things equal, the evolution 
of an animal that is large, or active, or both, implies mus-. 
cular strains that must tend to cause modification in such a 
cartilaginous rod. The results of greater bulk and of greater’ 
vivacity may be best dealt with separately. As the 
animal increases in size, the rod will grow both longer and 
thicker. On looking back at the diagrams of forces caused: 
by transverse strains, it will be seen that as the rod grows. 
thicker, its outer parts must be exposed to more severe ten- 
sions and pressures, if the degree of bend is the same. It is 
doubtless true that when the fish or reptile, advancing by- 
lateral undulations, becomes longer, the curvature assumed ° 
by the body at each movement becomes less; and that from: 
this cause the outer parts of the notochord are, other things: 
equal, less strained—the two changes thus partially neutral-° 
izing one another. But other things are not equal. For’ 
while, supposing the shape of the body to remain con- 
stant, the force exerted in moving the body increases as the 
cubes of its dimensions, the sectional area of the notochord, 
on which fall the reactions of this exerted force, increases - 
only as the squares of the dimensions: whence results an. 
intenser stress upon its substance. » Merely noting that the: 
other varying factor—the resistance of the water—may here. 


THE SHAPES OF VERTEBRATE SKELETONS. 202 


be left out of the account (since for similar masses moving 
with equal velocities the resistances increase but little faster 
than the squares of the dimensions, which is the rate at which 
the sectional areas of the notochords increase) we see that aug- 
menting bulk, taken alone, involves but a moderate residuary 
increase of strain on each portion of the notochord; and 
this is probably the reason why it is possible for a large s/ug- 
gish fish like the Sturgeon, to retain the notochordal struc- 
ture. But now, passing to the effects of greater ac- 
tivity, a like dynamical inquiry at once shows us how rapidly 
the violence of the actions and reactions rises as the move- 
ments become more vivacious. In the first place, the resist- 
ance of a medium such as water increases as the square of 
the velocity of the body moving through it; so that to main- 
tain double the speed, a fish has to expend four times the 
energy. But the fish has to do more than this—it has to 
initiate this speed, or to impress on its mass the force implied 
by this speed. Now the vis viva of a moving body varies as 
the square of the velocity ; whence it follows that the energy 
required to generate that vis viva is measured by the square 
of the velocity it produces. Consequently, did the fish put 
itself in motion instantaneously, the expenditure of energy in 
generating its own vis viva and simultaneously overcoming 
the resistance of the water, would vary as the fourth power 
of the velocity. But the fish cannot put itself in motion 
instantaneously—it must do it by increments; and thus it 
results that the amounts of the forces expended to give itself 
different velocities must be represented by some series of 
numbers falling between the squares and the fourth powers 
of those velocities. Were the increments slowly accumulated, 
the ratio of increasing effort would but little exceed the ratio 
of the squares; but whoever observes the sudden, convulsive 
action with which an alarmed fish darts out of a shallow into 
‘deep water, will see that the velocity is very rapidly gener- 
ated, and that therefore the ratio of increasing effort probably 
‘exceeds the ratio of the squares very considerably. At any 


204 _ MORPHOLOGICAL DEVELOPMENT. 


rate it will be clear that the efforts made by fish in rushing 
upon prey or escaping enemies (and it is these extreme efforts 
which here concern us) must, as fish become more active, 
rapidly exalt the strains to be borne by their motor organs ; 
and that of these strains, those which fall upon the noto- 
chord must be exalted in proportion to the rest. Thus the 
development of locomotive power, which survival of the 
fittest must tend in most cases to favour, involves such in- 
crease of stress on the primitive cartilaginous rod as will 
tend, other things equal, to cause its modification. 

What must its modification be P Considering the compli- 
cation of the influences at work, conspiring, as above indi- 
cated, in various ways and degrees, we cannot expect to do 
more than form an idea of its average character. The nature 
of the changes which the notochord is likely to undergo, where 
greater bulk is accompanied by higher activity, is. rudely 
indicated by Figs. 291, 292, and 293. The successively 


398 
4GL 
os ORLA 
ae ge ai eR | 
7 We eee eae a 


thicker lines represent the successively greater strains to 
which the outer layers of tissue are exposed ; and the widen- 
ing inter-spaces represent the greater extensions which they 
have to bear when they become convex, or else the greater 
gaps that must be formed in them. Had these outer layers 
to undergo extension only, as on the convex side, continued 
natural selection might result in the formation of a tissue 
elastic enough to admit of the requisite stretching. But at 
each alternate bend, these outer layers, becoming concave, 
are subject to increased compression—a compression which 
they cannot withstand if they have become simply more 
extensible. To withstand this greater compression they must 
become harder as well as more extensible. How are 
these two requirements to be reconciled? If, as facts war- 
rant us in supposing, a formation of denser substance occurs 


THE SHAPES OF VERTEBRATE SKELETONS. 205 


at those parts of the notochord where the strain is greatest ; 
it is clear that this formation cannot so go on as to produce 
a continuous mass: the perpetual flexions must prevent this. 
If matter that will not yield at each bend, is deposited while 
the bendings are continually taking place, the bendings will 
maintain certain places of discontinuity in the deposit— 
places at which the whole of the stretching consequent on 
each bend will be concentrated. And thus the tendency will 
be to form segments of hard tissue capable of great resistance 
to compression, with intervals filled by elastic tissue capable 
of great resistance to extension—a vertebral column. 

And now observe how the progress of ossification is just 
such as conforms to this view. That centripetal develop- 
ment of segments which holds of the vertebrate animal as a 
whole, as, if caused by transverse strains, it ought to do, and 
which holds of the vertebral column as a whole, as it ought 
to do, holds also of the central axis. On the mechanical 
hypothesis, the outer surface of the notochord should be the 
first part to undergo induration, and that division into seg- 
ments that must accompany induration. And accordingly, 
in a vertebral column of which the axis is beginning to 
ossify, the centrums consist of bony rings inclosing a still- 
continuous rod of cartilage. 


§ 258. Sundry other general facts which the comparative 
morphology of the Vertebrata discloses, supply further con- 
firmation. Let us take first the structure of the skull. 

On considering the arrangement of the muscular flakes, or 
myocommata, in any ordinary fish that comes to table—an 
arrangement already sketched out in the Amphiowus—it is not 
difficult to see that that portion of the body out of which the 
head of the vertebrate animal becomes developed, is a por- 
tion which cannot subject itself to bendings in the same 
degree as the rest of the body. The muscles developed there 
must be comparatively short, and much interfered with by 
the pre-existing orifices. Hence the cephalic part will not 


206 MORPHOLOGICAL DEVELOPMENT. 


partake in any considerable degree of the lateral undula- 
tions; and there will not tend to arise in it any such distinct 
segmentation as arises elsewhere. We have here, then, an 
explanation of the fact, that from the beginning the develop- 
ment of the head follows a course unlike that of the spinal 
column; and of the fact that the segmentation, so far as it 
can be traced in the head, is most readily to be traced in the 
occipital region and becomes lost in the region of the face. 
Still more significantly, we have an explanation of the fact 
that the base of the skull, answering to the front end of the 
notochord, never betrays any sign of segmentation. This, 
which is absolutely at variance with the hypothesis of the 
transcendental anatomists, is in complete harmony with the 
foregoing hypothesis. or if, as we have seen, the segmenta- 
tion consequent on mechanical actions and reactions must 
progress from without inwards, affecting last of all the axis; 
and if, as we have seen, the region of the head is so circum- 
stanced that the causes of segmentation act but feebly even 
on its periphery; then, it is to be expected that its axis 
will not be segmented at all: that portion of the primitive 
notochord which is included in the head, having to un- 
dergo no lateral bendings, may ossify without division into 
segments. 

Of other incidental evidences supplied by comparative 
morphology, let me next refer to the supernumerary bones, 
which the theory of Goethe and Oken as elaborated by Prof. 
Owen, has to get rid of by gratuitous suppositions. In many 
fishes, for example, there are what have been called inter- 
neural spines and inter-hzemal spines. These cannot by any 
ingenuity be affiliated upon the archetypal vertebra, and they 
are therefore arbitrarily rejected as bones belonging to the 
exo-skeleton ; though in shape and texture they are similar 
to the spines between which they are placed. On the hy- 
pothesis of evolution, however, these additional bones are 
accounted for as arising under actions like those that gave 
origin to the bones adjacent to them. And similarly with 


THE SHAPES OF VERTEBRATE SKELETONS. 207 


such bones as those called sesamoid; together with others 
too numerous to name. 

Again, in the course of evolution, both as displayed in the 
Vertebrata generally and in each vertebrate embryo, three 
skeletons succeed one another—the membranous, the car- 
tilaginous, and the osseous. These substitutions take place 
variously and unsystematically. While one part of a skele- 
ton retains the membranous character, another part of the 
same skeleton has become cartilaginous. At the same time 
that certain components have become partially or completely 
ossified, other components continue cartilaginous or mem- 
branous. Further, though there is a general succession of 
these stages, the succession is not regularly maintained ; for 
in many cases bones are formed by the deposit of osseous 
matter in portions of the membranous skeleton, which thus 
do not pass through the cartilaginous stage. “ Nor,” says 
Prof. Huxley, ‘‘ does any one of these states ever completely 
obliterate its predecessor; more or less cartilage and mem- 
brane entering into the composition of the most completely 
ossified skull, and more or less membrane being discoverable 
in the most completely chondrified skull.”” And then, too, 
the processes of chondrification and ossification often proceed 
with but little respect for the pre-existing divisions; but 
severally may result in the establishment of two parts where 
there was before one, or one where there were before two. 
Now wholly incongruous as these facts are with the hypothe- 
sis of an archetypal skeleton, they are quite congruous with 
the mechanical ‘hypothesis. This shows us why, in the 
course of evolution, a feebly-resisting membranous structure 
came to be replaced by a more-resisting cartilaginous struc- 
ture, and this, again, by a still-more-resisting osseous struc- 
ture; and why, therefore, these successive stages succeed one 
another, as it seems so superfluously, in the vertebrate em- 
bryo. And it further shows us why there is irregularity in 
the succession; seeing that the varying mechanical ac- 
tions to which the varying habits of the Vertebrata have 


208 MORPHOLOGICAL DEVELOPMENT. 


exposed them, have involved variations in the process of 
solidification. 


§ 259. Of course the foregoing synthesis is to be taken 
simply as an adumbration of the process by which the verte- 
brate structure may have arisen through the continued actions 
of known agencies. The motive for attempting it has been 
two-fold. Having, as before said, given reasons for con- 
cluding that the segments of a vertebrate animal are not 
homologous in the same sense as those of an annulose animal 
or a phenogamic axis, it seemed needful to do something 
towards showing how they are otherwise to be accounted for ; 
and having here, for our general subject, the likenesses and 
differences among the parts of organisms, as determined by 
incident forces, it seemed out of the question to pass by the 
problem presented by the vertebrate skeleton, 

Leaving out all that is hypothetical, the general argument 
may be briefly presented thus:—The evolution from the 
simplest known vertebrate animal, of a powerful and active 
vertebrate animal, implies the development of a stronger 
internal fulcrum. The internal fulerum cannot be made 
stronger without becoming more dense. And it cannot be- 
come more dense while retaining its lateral flexibility, with- 
out becoming divided into segments. Further, in conformity 
with the general principles thus far traced, these segments 
must be alike in proportion as the forces to which they are 
exposed are alike, and unlike in proportion as these forces 
are unlike; and so there necessarily results that unity in 
variety by which the vertebral column is from the beginning 
characterized. Once more, we see that the explanation ex- 
tends to those innumerable and more-marked divergences 
from homogeneity, which vertebree undergo in the various 
higher animals. Thus, the production of vertebre, the pro- 
duction of likenesses among vertebree, and the production of 
unlikenesses among vertebre, are interpretable as parts of 


THE SHAPES OF VERTEBRATE SKELETONS. 209 


one general process, and as harmonizing with one general 
principle. 

Whether sufficient or insufficient, the explanation here 
given assigns causes of known kinds producing effects such 
as they are known to produce. It does not, as a solution of 
one mystery, offer another mystery of which no solution is 
to be asked. It does not allege a Platonic id¢€a, or fictitious 
entity, which explains the vertebrate skeleton by absorbing 
into itself all the inexplicability. On the contrary, it assumes 
nothing beyond agencies by which structures in general are 
moulded—agencies by which these particular structures are, 
indeed, notoriously modifiable. An ascertained cause of 
certain traits in vertebra and other bones, it extends to all 
other traits of vertebre; and at the time assimilates the 
morphological phenomena they present to much wider classes 
of morphological phenomena. 


VOL, @ 14 


CHAPTER XV1. 
THE SHAPES OF ANIMAL CELLS. 


§ 260. Amone animals as among plants, the laws of mor- 
phological differentiation must be conformed to by the mor- 
phological units, as well as by the larger parts and the wholes 
formed of them. It remains here to point out that the con- 
formity is traceable where the conditions are simple. 

_In the shapes assumed by those rapidly-multiplying cells 
out of which each animal is developed, there is a conspicuous 
subordination to the surrounding actions. 
Fig. 294 represents the cellular embryonic 
mass that arises by repeated spontaneous 
fissions. In it we see how the cells, origin- 
ally spherical, are changed by pressure 
against one another and against the limit- 
ing membrane; and how their likenesses 
and unlikenesses are determined by the likenesses and un- 
likenesses of the forces to which they are exposed. This fact 
may be thought scarcely worth pointing out. But it is 
worth pointing out, because what is here so obvious a con- 
sequence of mechanical actions, is in other cases a conse- 
quence of actions composite in their kinds and involved in 
their distribution. Just as the equalities and inequalities of 
dimensions among aggregated cells, are here caused by the 
equalities and inequalities among their mutual pressures in 
different directions ; so, though less manifestly, the equalities 


THE SHAPES OF ANIMAL CELLS. 211 


and inequalities of dimensions among other aggregated cells, 
are caused by the equalities and inequalities of the osmotic, 
chemical, thermal, and other forces besides the mechanical, 
to which their different positions subject them. 


§ 261. This we shall readily see on observing the or- 
dinary structures of limiting membranes internal and ex- 
ternal. In Fig. 295, is 
shown a much-magnified 
section of a papilla from 
the gum. The cells of 
which it is composed Yeas vies 
originate in its deeper G Tf TEX Bak 
part; and are at first DOK (an 
approximately spherical. 
Those of them which, as they develop, are thrust outwards by 
the new cells that continually take their places, have their 
shapes gradually changed. As they grow and successively 
advance to replace the superficial cells, when these exfoliate, 
they become exposed to forces that are more and more dif- 
ferent in the direction of the surface from what they are in 
lateral directions; and their dimensions gradually assume 
corresponding differences. 

Another species of limiting membrane, called cylinder- 
epithelium, is represented 
in Fig. 296. Though its 
mode of development is 
such as to render the © 
shapes of its cells quite 
unlike those of pavement- 
epithelium, as the above-described kind is sometimes called, 
its cells equally exemplify the same general truth. For the 
chief contrast which each of them presents, is the contrast 
between its dimension at right angles to the surface of the 
membrane, and its dimension parallel to that surface. 


It is needless for our present purpose to examine further 
; 14 * 


ON fs "Ash 
at 


212 MORPHOLOGICAL DEVELOPMENT. 


the evidence furnished by Histology; nor, indeed, would 
further examination of this evidence be likely to yield de- 
finite results. In the cases given above we have marked 
differences among the incident forces; and therefore have a 
chance of finding, as we do find, relations between these and 
differences of form. But the cells composing masses of 
tissue are severally subject to forces that are indeterminate ; 
and therefore the interpretation of their shapes is imprac- 
ticable. It must suffice that so far as the facts go they are 
congruous with the hypothesis. 


CHAPTER XVII. 
SUMMARY OF MORPHOLOGICAL DEVELOPMENT. 


§ 262. Tuart any formula should be capable of expressing 
# common character in the shapes of things so unlike as a 
tree and a cow, a flower and a centipede, is a remarkable 
fact; and is a fact which affords strong prima facie evidence 
of truth. For in proportion to the diversity and multiplicity 
of the cases to which any statement applies, is the probability 
that it sets forth the essential relations. Those connexions 
which remain constant under all varieties of manifestation, 
are most likely to be the causal connexions. 

Still higher will appear the likelihood of an alleged law of 
organic form possessing so great a comprehensiveness, when 
we remember that on the hypothesis of Evolution, there must 
exist between all organisms and their environments, certain 
congruities expressible in terms of their actions and reac- 
tions. The forces being, on this hypothesis, the causes of the 
forms, it is inferable, @ priori, that the forms must admit of 
generalization in terms of the forces; and hence, such a 
generalization arrived at @ posteriori, gains the further pro- 
bability due to fulfilment of anticipation. 

Nearer yet to certainty seems the conclusion thus reached, 
on finding that it does but assert in their special manifesta- 
tions, the laws of Evolution in general—the laws of that uni- 
versal re-distribution of matter and motion which holdthrough- 
out the totality of things, as well as in each of its parts. 


214 MORPHOLOGICAL DEVELOPMENT. 


It will be useful to glance back over the various minor 
inferences arrived at, and contemplate them in their ensem- 
ble from these higher points of view. 


§ 263. That process of integration which every plant dis- 
plays during its life, we found reason to think has gone on 
during the life of the vegetal kingdom as a whole. Proto- 
plasm into cells, cells into folia, folia into axes, axes into 
branched combinations—such, in brief, are the stages passed 
through by every shrub; and such appear to have been the 
stages .through which plants of successively-higher kinds 
have been evolved from lower kinds. Even among certain 
groups of plants now existing, we find aggregates of the first 
order passing through various gradations into aggregates of 
the second order—here forming small, incoherent, indefinite 
assemblages, and there forming large, definite, coherent 
fronds, Similar transitions are traceable through which 
these integrated aggregates of the second order pass into 
ageregates of the third order: in one species the unions of 
parent-fronds with the fronds that bud out from them, being 
temporary, and in another species such unions being longer 
continued; until, in species still higher, by a gemmation 
that is habitual and regular, there is produced a definitely- 
integrated aggregate of the third order—an axis bearing 
fronds or leaves. And even between this type and a type 
further compounded, a link occurs in the plants which cast 
off, in the shape of bulbils, some of the young axes they 
produce. As among plants, so among animals. A 
like spontaneous fission of cells ends here in separation, there 
in partial aggregation, while elsewhere, by closer combina- 
tion of the multiplying units, there arises a coherent and 
tolerably definite individual of the second order. By the 
budding of individuals of the second order, there are in some 
eases produced other separate individuals like them; in some 
cases temporary aggregates of such like individuals; and in 
other cases permanent aggregates of them: certain of which 


SUMMARY OF MORPHOLOGICAL DEVELOPMENT. ate 


become so definitely integrated that the individualities of 
their component members are almost lost in a tertiary indi- 
viduality. 

Along with this progressive integration there has gone on 
a progressive differentiation. Vegetal units of whatever 
order, originally homogeneous, have become heterogeneous 
while they have become united. Spherical cells aggregating 
into threads, into lamin, into masses, and into special tis- 
sues, lose their sphericity; and instead of remaining all 
alike assume innumerable unlikenesses—from uniformity pass 
into multiformity. Fronds combining to form axes, severally 
acquire definite differences between their attached ends and 
their free ends; while they also diverge from one another 
in their shapes at different parts of the axes they compose. 
And axes, uniting into aggregates of a still higher order, be- 
come more or less contrasted in their sizes, curvatures, and 
arrangements of their appendages. Similarly among 
animals. Those components of them which, with a certain 
license, we class as morphological units, while losing their 
minor individualities in the major individualities formed of 
them, grow definitely unlike as they grow definitely com- 
bined. And where the aggregates so produced become, by 
coalescence, segments of aggregates of a still higher order, 
they, too, diverge from one another in their shapes. 

The morphological differentiation which thus goes hand in 
hand with morphological integration, is clearly what the 
perpetually-complicating conditions would lead us to antici- 
pate. Every addition of a new unit to an aggregate of such 
units, must affect the circumstances of the other units in all 
varieties of ways and degrees, according to their relative 
positions—-must alter the distribution of mechanical strains 
throughout the mass, must modify the process of nutrition, 
must affect the relations of neighbouring parts to surround- 
ing diffused actions; that is, must initiate a changed inci- 
dence of forces tending ever to produce changed structural 
arrangements. 


216 MORPHOLOGICAL DEVELOPMENT. 


§ 264. This broad statement of the correspondence be- 
tween the general facts of Morphological Development and 
the principles of Evolution at large, may be reduced to state- 
ments of a much more specific kind. The phenomena of 
symmetry and unsymmetry and asymmetry, which we have 
traced out among organic forms, are demonstrably in har- 
mony with those laws of the re-distribution of matter and 
motion to which Evols tion conforms. Besides the myriad- 
fold illustrations of the instability of the homogeneous, that 
are afforded by these aggregates of units of each order, which, 
at first alike, lapse gradually into unlikeness; and besides 
the myriad-fold illustrations of the multiplication of effects, 
which these ever-complicating differentiations exhibit to us; 
we have also myriad-fold illustrations of the definite equal- 
ities and inequalities of structures, produced by definite 
equalities and inequalities of forces. 

The proposition arrived at when dealing with the causes 
of Evolution, “that in the actions and reactions of force and 
matter, an unlikeness in either of the factors necessitates an 
unlikeness in the effects; and that in the absence of unlike- 
ness in either of the factors the effects must be alike” (First 
Principles, § 129), is the general formula including all 
these particular likenesses and unlikenesses of parts which 
we have been tracing. For have we not everywhere seen 
that the strongest contrasts are between the parts that are 
most contrasted in their conditions; while the most similar 
parts are those most-similarly conditioned? In every plant 
the leading difference is between the attached end and the 
free end; in every branch it is the same; in every leaf it is 
the same. And in every plant the leading likenesses are 
those between the two sides of the branch, the two sides of 
the leaf, and the two sides of the flower, where these parts 
are two-sided in their conditions ; or between all sides of the 
branch, all, sides of the leaf, and all sides of the flower, where 
these parts are similarly conditioned on all sides. So, too, is 
it with animals that move about. The most marked contrasts 


SUMMARY OF MORPHOLOGICAL DEVELOPMENT. 217 


they present are those between the part in advance and the 
part behind, and between the upper part and the under part ; 
while there is complete correspondence between the two 
sides. Externally the likenesses and differences among 
limbs, and internally the likenesses and differences among 
vertebrz, are expressible in terms of this same law. 

And here, indeed, we may see clearly that these truths are 
corollaries from that ultimate truth to which all phenomena 
of Evolution are referable. It is an inevitable deduction 
from the persistence of force, that organic forms which have 
been progressively evolved must present just those funda- 
mental traits of form which we find them present. It cannot 
but be that during the intercourse between an organism and 
its environment, equal forces acting under equal conditions 
must produce equal effects; for to say otherwise, is, by im- 
plication, to say that some force can produce more or less 
than its equivalent effect, which is to deny the persistence of 
force. Hence those parts of an organism which are, by its 
habits of life, exposed to like amounts and like combinations 
of actions and reactions, must develop alike; while unlike- 
nesses of development must as unavoidably follow unlike- 
“nesses among these agencies. And this being s0, all the 
specialities of symmetry and unsymmetry and asymmetry 
which we have traced, are necessary consequences. 


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PART. V. 
PHYSIOLOGICAL DEVELOPMENT. 


CHAPTER I. 


THE PROBLEMS OF PHYSIOLOGY. 


§ 265. THE questions to be treated under the above 
title are widely different from those which it ordinarily 
expresses. We have no alternative, however, but to use 
Physiology in a sense co-extensive with that in which we 
have used Morphology. We must here consider the facts of 
function in a manner parallel to that in which we have, 
in the foregoing Part, considered the facts of structure. 
As, hitherto, we have concerned ourselves with those most 
general phenomena of organic form which, holding irre- 
spective of class and order and sub-kingdom, illustrate the 
processes of integration and differentiation characterizing 
Evolution in general ; so, now, we have to concern ourselves 
with the evidences of those differentiations and integrations 
of organic functions which have simultaneously arisen, and 
which similarly transcend the limits of zoological and 
botanical divisions. How heterogeneities of action have 
progressed along with heterogeneities of structure—that is 
the inquiry before us; and obviously, in pursuing it, all 
the specialities with which Physiology usually deals can 
serve us only as materials. 

Before entering on the study of Morphological Develop- 
ment, it was pointed out that while facts of structure may 
be empirically generalized apart from facts of function, they 


222 PHYSIOLOGICAL DEVELOPMENT. 


cannot be rationally interpreted apart; and throughout the 
foregoing pages this truth has been made abundantly mani- 
fest. Here we are obliged to recognize the inter-dependence 
still more distinctly; for the phenomena of function cannot 
even be conceived without direct and perpetual consciousness 
of the phenomena of structure. Though the subject-matter 
of Physiology is as broadly distinguished from the subject- 
matter of Morphology as motion is from matter; yet, just 
as the laws of motion cannot be known apart from some 
matter moved, so there can be no knowledge of function 
without a knowledge of some structure as performing func- 
tion. 

Much more than this is obvious. The study of functions, 
considered from our present point of view as arising by 
Evolution, must be carried on mainly by the study of the cor- 
relative structures. Doubtless, by experimenting on the organ- 
isms that are growing and moving around us, we may 
ascertain the connexions existing among certain of their 
actions, while we have little or no knowledge of the special 
parts concerned in those actions. In a living animal that 
can be conveniently kept under observation, we may learn 
the way in which conspicuous functions vary together—how 
the rate of a man’s pulse increases with the amount of 
muscular exertion he is undergoing; or how a horse’s 
rapidity of breathing is in part dependent on his speed. 
But though observations of this order are indispensable— 
though by accumulation and comparison of such obser- 
vations we learn which parts perform which functions— 
though such observations, prosecuted so as to disclose 
the actions of all parts under all circumstances, constitute, 
when properly generalized and co-ordinated, what is com- 
monly understood as Physiology; yet such observations 
help us but a little way towards learning how functions 
came to be established and specialized. We have next to 
no power of tracing up the genesis of a function considered 
purely as a function—no opportunity of observing the 


THE PROBLEMS OF PHYSIOLOGY. 223 


progressively-increasing quantities of a given action that 
have arisen in any order of organisms. In nearly all cases 
we are able only to establish the greater growth of the part 
which we have found performs the action, and to infer that 
greater action of the part has accompanied greater growth 
of it. The tracing out of Physiological Development, then, 
becomes substantially a tracing out of the development of 
the organs by which the functions are known to be dis- 
charged—the differentiation and integration of the functions 
being presumed to have progressed hand in hand with the 
differentiation and integration of the organs. Between the 
inquiry pursued in Part IV, and the inquiry to be pursued 
in this Part, the contrast is that, in the first place, facts of 
structure are now to be used to interpret facts of function, 
instead of conversely; and, in the second place, the facts 
of structure to be so used are not those of conspicuous shape 
so much as those of minute texture and chemical com- 
position. 


§ 266. The problems of Physiology, in the wide sense 
above described, are, like the problems of Morphology, to be 
considered as problems to which answers must be given in 
terms of incident forces. On the hypothesis of Evolution 
these specializations of tissues and accompanying concen- 
trations of functions, must, like the specializations of shape 
in an organism and its component divisions, be due to the 
actions and reactions which its intercourse with the environ- 
ment involves; and the task before us is to explain how they 
are wrought—how they are to be comprehended as results 
of such actions and reactions. 

Or, to define these problems still more specifically :—Those 
extremely unstable substances which compose the proto- 
plasm of which organisms are mainly built, have to be 
traced through the various modifications in their properties 
and powers, that are entailed on them by changes of relation to 
agencies of all kinds. Those organic colloids which pass from 


224 PHYSIOLOGICAL DEVELOPMENT. 


liquid to solid and from soluble to insoluble on the slightest 
molecular disturbance—those albumenoid matters which, as 
we see in clotted blood or the coaguiable lymph that is 
poured out on abraded surfaces and causes adhesion between 
inflamed membranes, assume new forms with the greatest 
readiness, are to have their metamorphoses studied in con- 
nexion with the influences at work. Those compounds which, 
as we see in the quickly-acquired brownness of a bitten 
apple or in the dark stains produced by the milky juice of a 
Dandelion, immediately begin to alter when the surrounding 
actions alter, are to be everywhere considered as undergoing 
modifications by modified conditions. Organic bodies, con- 
sisting of substances that, as I here purposely remind the 
reader, are prone beyond all others to change when the 
incident forces are changed, we must contemplate as in all 
their parts differently changed in response to the different 
changes of the incident forces. And then we have to re- 
gard the concomitant differentiations of their reactions as 
being concomitant differentiations of their functions. 

Here, as before, we must take into account two classes 
of factors. We have to bear in mind the inherited results of 
actions to which antecedent organisms were exposed, and to - 
join with these the results of present actions. Hach organism 
is to be considered as presenting a moving equilibrium of 
functions, and a correlative arrangement of structures, 
produced by the aggregate of actions and reactions that have 
taken place between all ancestral organisms and their envi- 
ronments. The tendency in each organism to repeat this 
adjusted arrangement of functions and structures, must be 
regarded as from time to time interfered with by actions to 
which its inherited equilibrium is not adjusted—actions to 
which, therefore, its equilibrium has to be re-adjusted. And 
in studying physiological development we have in all cases 
to contemplate the progressing compromise between the old 
and the new, ending in a restored balance or adaptation. 

It is manifest that our data are so scanty that nothing 


THE PROBLEMS OF PHYSIOLOGY. 925 


more than very general and approximate interpretations of 
this kind are possible. If the hypothesis of Evolution fur- 
nishes us with a rude conception of the way in which the 
more conspicuous and important differentiations of functions 
have arisen, it is as much as can be expected. 


§ 267. It will be best, for brevity and clearness, to deal 
with these physiological problems as we dealt with the 
morphological ones—to carry on the inductive statement and 
the deductive interpretation hand-in-hand: so disposing 
of each general truth before passing to the next. Treating 
separately vegetal organisms and animal organisms, we will 
in each kingdom consider:—first, the physiological differentia- 
tions and accompanying changes of structure that arise be- 
tween outer tissues and inner tissues; next, those that arise 
between different parts of the outer tissues; and, finally, 
those that arise between different parts of the inner tissues. 
What little has to be said concerning physiological integra- 
tion must come last. For though, in tracing up Mor- 
phological Evolution, we have to study those processes of 
integration by which organic aggregates are formed, before 
studying the differentiations that arise among their parts ; 
we must, contrariwise, in tracing up Physiological Evolution, 
study the genesis of the different functions before we study 
the inter-dependence that eventually arises among them and 
constitutes physiological unity. 


VoL i, 15 


CHAPTER II. 


DIFFERENTIATIONS BETWEEN THE OUTER AND INNER 
TISSUES OF PLANTS. 


§ 268. The simplest plant presents a contrast between its 
peripheral substance and its central substance. In each pro- 
tophyte, be it a spherical cell or a branched tube, or such 
a more-specialized form as a Desmid, a marked unlikeness 
exists between the limiting layer and that which it limits. 
These vegetal aggregates of the first order may differ widely 
from one another in the natures of their outer coats and in 
the natures of their contents. Asin a Palmella, there may 
exist a clothing of jelly; or, as in Diatom, the walls may 
take the form of silicious valves variously sculptured. The 
contained matter may be here green, there red, and in other 
eases brown or black. But amid all these diversities there is 
this one uniformity —a strong distinction between the parts 
in contact with the environment and the parts not in con- 
tact with the environment. 

When we remember that this trait is one which these 
simple living bodies have in common with bodies that are not 
living —when we remember that each inorganic mass even- 
tually has its outer part more or less differentiated from 
its inner part, here by oxidation, there by drying, and else- 
where by the actions of light, of moisture, of frost; we can 
scarcely resist the conclusion that, in the one case as in 


THE OUTER AND INNER TISSUES OF PLANTS. 223i 


the other, the contrast is due to the unlike actions to which 
the parts are subject. Given an originally-homogenous 
portion of protoplasm, and it follows from the general laws 
of Evolution (First Principles, §§ 109—115) first, that it must 
lose its homogeneity, and, second, that the leading dissimila- 
rities must arise between the parts most-dissimilarly con- 
ditioned—that is, between the outside and the inside. The 
exterior must bear amounts and kinds of force unlike the 
amounts and kinds which the interior bears; and from 
the persistence of force it follows inevitably that unlike 
effects must be wrought on them—they must be differen- 
tiated. 

What is the limit towards which the differentiation 
tends? We have seen that the re-distribution of matter 
and motion whence, under certain conditions, evolution 
results, can never cease until equilibrium is reached—proxi- 
mately a moving equilibrium, and finaily a complete equi- 
librium (First Principles, 8§ 180—135). Hence, the diffe- 
rentiation must go on. until it establishes such differences in 
the parts as shall balance the differences in the forces acting 
on them. When dealing with equilibration in general, we 
saw that this process is what is called adaptation (Mist 
Principles, § 133) ; and, more recently, we saw that by it the 
totality of functions of an organism is brought into cor- 
respondence with the totality of actions affecting it (§§ 159 
—163). Manifestly in this case, as in all others, either 
death or adjustment must eventually result. A force falling 
on one of these minute aggregates of protoplasm, must ex- 
pend itself in working its equivalent of change. If this 
force is such that in expending itself it disturbs beyond 
rectification the balance of the organic processes, then the 
aggregate is disintegrated or decomposed. But if it does 
not overthrow that moving equilibrium constituting the life 
of the aggregate, then the aggregate continues in that modi- 
fied form produced by the expenditure of the force. Thus, 


by direct equilibration, continually furthered by indirect 
bs 


228 PHYSIOLOGICAL DEVELOPMENT. 


equilibration, there must arise this distinction between the 
outer part adapted to meet outer forces, and the inner part 
adapted to meet inner forces. And their respective actions, 
as thus meeting outer and inner forces, must be what we call 
their respective functions. 


§ 269. Ageregates of the second order exhibit parallel 
traits, admitting of parallel interpretations. Integrated 
masses of cells or units homologous with protophytes, 
habitually show us contrasts between the characters of the 
superficial tissues and the central tissues. Such among these 
ageregates of the second order as have their component units 
arranged into threads or lamine, single or double, cannot, of 
course, furnish contrasts of this kind; for all their units are 
as much external as internal. We must turn to the more or 
less massive forms. | 

Of these, among Fungi, the common Puff-ball is a good 
example—good because it presents this fundamental differen- 
tiation but little complicated by others. In it we have a 
cortical layer of cellular tissue obviously unlike the mass of 
cellular tissue which it incloses. So far as the unlikeness 
between external and internal parts is concerned, we see here 
a relation analogous to that existing in the simple cell; and 
we see in it a similar meaning: there is a physiological 
differentiation corresponding to the difference in the incidence 
of forces. 

Under various forms the Alge show just the same rela- 
tion. Where, as in Codium Bursa, we have ramified tubular 
cells aggregated into a hollow globular mass, the outer and 
inner surfaces are contrasted both in colour and structure; 
though the tubules composing the two surfaces are con- 
tinuous with one another. In Rivularia, again, we see the 
like, both in the radial arrangement of the imbedded threads 
and in the difference of colour between the exterior of the 
imbedding jelly and its interior. The more-developed A/g@ 
of all kinds repeat the antithesis. In branched stems, 


THE OUTER AND INNER TISSUES OF PLANTS. 229 


when they consist of more than single rows of cells, the 
outer cells become unlike the inner, as shown in Fig. 35. 
Such types as Chrysymenia rosea show us this unlikeness 
very conspicuously. And it holds even with ribbon-shaped 
fronds. Wherever one of these is composed of three, four, 
or more layers, as in Laminaria and Punctaria, the cells of 
the external layers are strongly distinguished from those of 
the internal layers, both by their comparative smallness and 
by their deep colour. 


§ 270. The higher plants variously display the like 
fundamental distinction between outer and inner tissues. 
Each leaf, thin as it is, exemplifies this differentiation of the 
parts immediately in contact with the environment from the 
parts not in immediate contact with the environment. Its 
cuticular cells, forming a protecting envelope, diverge physi- 
eally and chemically from the contained cells of parenchyma, 
which carry on the more active functions. And the contrast 
may be observed to establish itself in the course of develop- 
ment. At first the component cells of the leaf are all alike ; 
and this unlikeness between the cells of the outer and inner 
layers, arises simultaneously with the rise of differences in 
their conditions—differences that have acted on all ancestral 
leaves as they act on the individual leaf. 

An unlikeness more marked in kind but similar in mean- 
ing, exists between the bark of every branch and the tissues 
it clothes. The phenogamic axis, especially when exogenous, 
is commonly characterized by an outer layer differing from 
the inner layers in character and function, as it differs from 
them in position. Subject as this outer layer is to the un- 
mitigated actions of forces around—to abrasions, to extremes 
of heat and cold, to evaporation and soaking with water—its 
units cannot cease changing until they are in equilibrium 
with these more violent actions, and have acquired. molecular 
constitutions more stable than those of the interior cells. 
That is to say, the forces which differentiate the cortical part 


230 PHYSIOLOGICAL DEVELOPMENT. 


from the rest are the forces which it has to resist, and from 
which it passively protects the parts within. How 
clearly this heterogeneity of structure and function is conse- 
quent upon intercourse with the environment, every tree 
and shrub shows. The young shoots, alike of annuals and 
perennials, are quite green and soft at their extremities. 
Among plants of short lives, there is usually but a slight 
development of bark: such traces of it as the surface of the 
axis acquires being seen only at its lowermost or oldest 
portion. In long-lived plants, however, this formation of a 
tough opaque coating takes place more rapidly; and shows 
us distinctly the connexion between the degree of differentia- 
tion and the length of exposure. For, in a growing twig, 
we see that the bark, invisible at the bud, thickens by 
insensible gradations as we go downwards to the junction of 
the twig with the branch ; and we come to still thicker parts 
of it as we descend along the branch towards the main stem. 
Moreover, on examining main stems we find that while in 
some trees the bark, cracked by expansion of the wood, drops 
off in flakes, leaving exposed patches of the inner tissue which 
presently become green and finally develop new cortical 
layers ; in other trees the exfoliated flakes continue adherent, 
and in the course of years form a rugged fissured coat: so 
producing a still more marked contrast between outside and 
inside. Of course the establishment of this he- 
terogeneity is furthered by natural selection, which, where a 
protective covering is needed, gives an advantage to those 
individuals in which it has become strongest. But that this 
divergence of structure commences as a direct adaptation, is 
clearly shown by other facts than the foregoing. There is 
the fact that many of the plants which in our gardens 
develop bark with considerable rapidity, do not develop it 
with the same rapidity in a greenhouse. And there is the 
fact that plants which, in some climates, have their stems 
covered only by thin semi-transparent layers, acquire thick 
opaque layers when taken to other climates. 


THE OUTER AND INNER TISSUES OF PLANTS. 231 


Just noting, for the sake of completeness, that in the 
roots of the higher plants there arises a contrast between 
outer and inner parts, parallel to the one we have traced in 
their branches, let me draw attention to another differentia- 
tion of the same ultimate nature, which the higher plants 
exhibit to us—a differentiation which, familiar though it is, 
gains a new meaning by association with those named above, 
and makes their meaning still more manifest. I refer to the 
fact that when, by the budding of axes out of axes, there 
is produced one of those highly-compounded Phenogams 
which we call a tree, the central part of the aggregate be- 
comes functionally and structurally unlike the peripheral 
part. On looking into a large tree, or even a small one 
that has thick foliage, like the Laurel, we see that the in- 
ternal branches are almost or quite bare of leaves, while the 
leaf-clad branches form an external stratum; and all our 
experience unites in proving that this contrast arises by 
degrees, as fast as the growth of the tree entails a contrast be- 
tween the conditions to which inner and outer branches are 
exposed. Now when, in these most-composite aggregates, 
we see a differentiation between peripheral and central parts 
demonstrably caused by a difference in the relations of these 
parts to environing forces, we get support for the conclusion 
otherwise reached, that there is a parallel cause for the parallel 
differentiations exhibited by all aggregates of lower orders-—— 
branches, leaves, cells. 


§ 271. Before leaving this most general physiological 
differentiation, it may be well to say something respecting 
certain secondary unlikenesses that habitually arise be- 
tween interior and exterior. For the contrast is not, as 
might be supposed from the foregoing descriptions, a simple 
contrast: it is a compound contrast. The outer structure 
itself is usually divisible into concentric structures. This 
is equally true of a protophyte and of a phenogamic axis. 
Between the centre of an independent vegetal cell and its 


232 PHYSIOLOGICAL DEVELOPMENT. 


surface, there are at least two layers; and the bark coating 
the substance of a shoot, besides being itself compound, 
includes another tissue lying between it and the wood. What 
is the physical interpretation of these facts ? 

When a mass of what we distinguish as inert matter is 
exposed to external-agencies capable of working changes in 
it—when it is chemically acted upon, or when, being dry, it 
is allowed to soak, or when, being wet, it is allowed to dry— 
the changes set up progress in an equable way from the 
surface towards the centre. At any time during the process 
(supposing no other action supervenes) the modification 
wrought, first completed at the outside, either gradually 
diminishes as we approach the centre, or ceases suddenly 
at a certain distance from the centre. But now suppose that 
the mass, instead of being inert, is the seat of active changes 
—suppose that it is a portion of complex colloidal substance, 
permeable by light and by fluids capable of affecting its 
unstable molecules—suppose that its interior is a source of 
forces continually liberated and diffusing themselves out- 
wards. Is it not likely that while at the centre the action 
of the internally-liberated forces will dominate, and while at 
the surface the action of the environing forces will dominate, 
there will be between the two a certain place at which their 
actions balance? And may we not expect that this will be 
the place where the most unstable matter exists—the place 
outside of which the matter becomes relatively stable in the 
face of external forces, and inside of which the matter be- 
comes relatively stable in the face of internal forces ? 

Be this or be this not the explanation, the well-known fact 
is that the inner wall of each vegetal cell is a delicate mem- 
brane, the primordial utricle, composed of that nitrogenous 
substance specially characterized by instability; and that 
outside of this is the cellulose layer, and inside of it the 
granular colouring matter. And the similarly well-known 
fact is, that in each phenogamic axis the cambium layer, 
which shows its relative instability by the activity of the 


THE OUTER AND INNER TISSUES OF PLANTS. 233 


changes going on in it, lies between the bark and the mass 
of the axis; and is the place from which the differentiations 
producing these proceed in opposite directions. 

But we are here chiefly concerned with the more general 
interpretation, which is independent of any such speculation 
as the foregoing. These contrasted tissues and the contrasted 
functions they severally perform are, beyond question, sub- 
ordinated to the relations of outside and inside. And the 
evidence makes it tolerably clear that the unlike actions of 
forces involved by the relations of outside and inside, deter- 
mine these contrasts—partly directly and partly indirectly. 


CHAPTER Iii. 


DIFFERENTIATIONS AMONG THE OUTER TISSUES OF PLANTS. 


§ 272. The Protococct and such compound forms as the 
Volvox globator, which do not permanently expose any parts 
of their surfaces to actions unlike those which other parts 
are exposed to, have no parts of their surfaces unlike the 
rest in function and composition. This is what the hypo- 
thesis prepares us for. If physiological differentiations are 
determined by differences in the incidence of forces, then 
there will be no such differentiations where there are no 
such differences. Contrariwise, it is to be expected that the 
most conspicuous unlikeness of function and minute structure 
will arise between the most-dissimilarly circumstanced parts 
of the surface. We find that they do. The upper end and the 
lower end, or, more strictly speaking, the free end and the 
attached end, habitually present the strongest physiological 
contrasts. 

Even aggregates of the first order illustrate this truth. 
Such so-called unicellular plants as those delineated in 
Figs 4, 5, and 6, show us, on comparing the contents of 
their fixed ends aud their loose ends, that different processes 
are going on in them, and that different functions are 
being performed by their limiting membranes. Caulerpa 
prolifera, which “consists of a little creeping stem with 
roots below and leaves above,” originating “in the 


THE OUTER TISSUES OF PLANTS. 935 


growth of a body which may be regarded as an individual 
cell,” supplies a still-better example. Among 
ageregates of the second order the like connexion is 
displayed in more various modes but with equal con- 
sistency. As, before, the Puff-ball served to exemplify the 
primary physiological differentiation of outer parts from 
inner parts; so, here, it supplies a simple illustration of 
the way in which the differentiated outer part is re-dif- 
ferentiated, in correspondence with the chief contrast in its 
relations to the environment. The only marked unlikeness 
which the cortical layer of the Puff-ball presents, is that 
between the portion next the ground and the opposite portion. 
The better-developed Fungi exhibit a more decided hetero- 
geneity of parallel kind. Such incrusting Alge as Ralfsia 
deusta furnish a kindred contrast; and in the higher Alge 
it is uniformly repeated. Pheenogams display 
this physiological differentiation very conspicuously. That 
earth and air are unlike portions of the environment, sub- 
jecting roots and leaves to unlike physical forces, which entail 
on them unlike reactions; and that the unlike functions and 
structures of their respective surfaces are fitted to these 
unlike physical forces; are familiar facts which it would be 
needless here to name, were it not that they must be counted 
as coming within a wider group of facts. 

Is this unlikeness between the outer tissues of the attached 
ends and those of the free ends in plants, determined by 
their converse with the unlike parts of the environment ? 
That they result from an equilibration partly arising in 
the individual and partly arising by the survival of indivi- 
duals in which it has been carried furthest, is inferable 
ad priori; and this @ priori argument may be adequately 
enforced by arguments of the inductive order. A few 
typical ones must here suffice. The gemmules 
of the Muarchantia are little disc-shaped masses of cells 
composed of two or more layers. Their sides being alike, 
there is nothing to determine which side falls lowermost 


236 PHYSIOLOGICAL DEVELOPMENT. 


when one of them is detached. Whichever side falls lower- 
most, however, presently begins to send out rootlets, while the 
uppermost side begins to assume those characters which 
distinguish the face of the frond. When this differentiation 
has commenced, the tendency to its complete establishment 
becomes more and more decided; as is proved by the fact 
that if the positions of the surfaces be altered, the gemmule 
bends itself so as to re-adjust them: the change towards 
equilibrium with environing forces having been once set up, 
there is acquired, as it were, an increasing momentum which 
resists any counter-change. But the evidence shows that 
at the outset, the relations to earth and air alone deter- 
mine the differentiation of the under surface from the 
upper. The experiences of the gardener, multi- 
plying his plants by cuttings and layers, constitute another 
class of evidences not to be omitted: they are commonplace 
but instructive examples of physiological differentiation. 
While circumstanced as it usually is, the cambium of each 
branch in a Pheenogam continues to perform its ordinary 
function—transforming itself on its outer side imto the 
cortical substances, and on its inner side into vascular and 
woody tissues. But change the conditions to those which 
the underground part of the plant is exposed to, and there 
begins another differentiation resulting in underground struc- 
tures. Contact with water often suffices alone to produce 
this result, as in the branches of some trees when they droop 
into a pool, or as occasionally with a cutting placed in a 
bottle of water; and when the light is excluded by im- 
bedding the end of the cutting, or the middle of the still- 
attached branch, in the earth, this production of. tissues 
adapted to the function of absorbing moisture and mineral 
constituents proceeds still more readily. With such cases 
may be grouped those in which this development of under- 
ground organs by an above-ground tissue, is not excep- 
tional but habitual. Creeping plants furnish good illus- 
trations. From the shoots of the Ground-Ivy, rootlets are 


THE OUTER TISSUES OF PLANTS. 237 


put out into the soil in a manner differing but little from 
that in which they are put out by an imbedded layer; save 
that the process follows naturally-induced conditions instead 
of following artificially-induced conditions. But in the 
common Ivy which, instead of running along the surface 
of the earth, runs up inclined or vertical surfaces, we see the 
process interestingly modified without being essentially 
changed. The rootlets, here differentiated by their con- 
ditions into organs of attachment much more than organs of 
absorption, still develop on that side of the shoot next the 
supporting surface, and do not develop where the shoot, 
growing away from the tree or wall, is surrounded equally 
on all sides by light and air—thus showing, undeniably, 
that the production of the rootlets is determined by the 
differential incidence of forces. That greenness 
which may be observed in these Ivy-branch rootlets while 
they are quite young, soft, and unshaded, introduces us to 
facts which are the converse of the foregoing facts ; and prove 
that the parts ordinarily imbedded in the soil and adapted to 
its actions, acquire, often in a very marked degree, the super- 
ficial structures of the aérial parts, when they are exposed to 
light and air. This may be witnessed in Maize, which, when 
luxuriant, sends out from its nodes near the ground, clusters 
of roots that are thick, succulent, and of the same colour as 
the leaves. Examples more familiar to us in England, occur 
in every field of Turnips. On noting how green is the un- 
covered part of a Turnip-root, and how manifestly the 
area over which the greenness extends varies with the area 
exposed to light, as well as with the degree of the exposure, 
it will be seen that beyond question, root-tissue assumes 
to a considerable extent the appearance and function of 
leaf-tissue, when subject to the same agencies. Let+us not 
forget, too, that where exposed roots do not approach in 
superficial character towards leaves, they approach in 
superficial character towards stems—becoming clothed 
with a thick, fissured bark, like that of the trunk and 


238 | PHYSIOLOGICAL DEVELOPMENT. 


branches. But the most conclusive evidence is 
furnished by the actual substitutions of surface-structures 
and functions, that occur in aérial organs which have taken 
to growing permanently under ground, and in under-ground 
organs which have taken to growing permanently in the 
air. On the one hand, there is the rhizoma, exemplified by 
Ginger—a stem which, instead of shooting up vertically, 
runs horizontally below the surface of the soil, and assumes 
the character of a root, alike in colour, texture, and 
production of rootlets; and there is that kind of swollen 
under-ground axis, bearing axillary buds, which the Potato 
exemplifies—a structure which, though homologically an 
axis, simulates a tuberous root in surface-character, and 
when exposed to the air, manifests no greater readiness 
to develop chlorophyil than a tuberous root does. On the 
other hand, there are the aérial roots of certain Orchids, 
which, habitually green at their tips, continue green 
throughout their whole lengths when kept moist; which 
have become leaf-like not only by this development of 
chlorophyll, but also by the acquirement of stomata; and 
which do not bury themselves in the soil when they have 
the opportunity. Thus we have aérial organs so com- 
pletely changed to fit under-ground actions, that they will 
not resume aérial functions; and under-ground organs so 
completely changed to fit aérial actions, that they will not 
resume under-ground functions. 

That the physiological differentiation between the part of 
a plant’s surface which is exposed to light and air and the 
part which is exposed to darkness and moisture and solid 
matter, is primarily due to the unlike actions of these 
unlike parts of the environment, is, then, clearly implied by 
observed facts—more clearly, indeed, than was to be expected. 
Considering how strong must be the inherited tendency 
of a plant to assume those special characters, physio- 
logical as well as morphological, which have resulted from 
an enormous accumulation of antecedent actions, it may 


THE OUTER TISSUES OF PLANTS. 239 


be even thought surprising that this tendency can 
be counteracted to so great an extent by changed con- 
ditions. Such a degree of modifiability becomes compre- 
hensible, only when we remember how little a plant’s 
functions are integrated; and how much, therefore, the 
functions going on in each part may be altered without 
having to overcome the momentum of the functions through- 
out the whole plant. But this modifiability being as great 
as it is, we can have no difficulty in understanding how, by 
the cumulative aid of natural selection, this primary differen- 
tiation of the surface in plants has become what we see it. 


§ 273. We will leave now these contrasts between the 
free surfaces of plants and their attached or imbedded 
surfaces, and turn our attention to the secondary contrasts 
existing between different parts of their free surfaces. Were 
a full statement of the evidence practicable, it would be 
proper here to dwell on that which is furnished by the 
inferior classes. It might be pointed out in detail that 
where, as among the Algae, the free surfaces are not dis- 
similarly conditioned, there is no systematic differentiation of 
them—that the frond of an Ulva, the ribbon-shaped divisions 
of a Laminaria, and the dichotomous expansions of the Fuci 
that clothe the rocks between tide-marks, are alike on both 
-sides; because, swayed about in all directions as they are by 
the waves and tides, their sides are equally affected. Con- 
versely, from the Fungi might be drawn abundant proof that 
even among Thallogens, unlikenesses arise between different 
parts of the free surfaces when their circumstances are unlike 
—that in such laterally-growing kinds as are shown in Fig. 
196, the honeycombed under surface and the smooth 
leathery upper surface, have their contrasts related to con- 
trasted conditions; and that in the adjacently-figured 
Agarics, and other stalked genera, the pileus exhibits a 
parallel difference, explicable in a parallel way. But passing 
over Cryptogams, it must suffice if we examine more at 


240 PHYSIOLOGICAL DEVELOPMENT. 


length these traits as they are displayed by Phenogams. Let 
us first note the dissimilarities between the outer tissues of 
stems and leaves. ‘ 

That these dissimilarities arose by degrees, as fast as the 
units of which the phenogamic axis is composed became 
integrated, is a conclusion in harmony with the truth that in 
every shoot of every plant, they are at first slight and become 
gradually marked. Already, in briefly tracing the contrasts 
between the outer and inner tissues of plants, some facts 
have been named showing, by implication, how the cessa- 
tion of the leaf-function in axes is due to that change of 
conditions entailed by the discharge of other functions. . Here 
we have to consider more closely facts of this class, together 
with others immediately to the point. On pulling 
off from a stem of grass the successive sheaths of its leaves, 
the more-inclosed parts of which are of a fainter green than 
the outer parts, it will be found that the tubular axis even- 
tually reached is of a still fainter green ; but when the axis 
eventually shoots up into a flowering stem, its exposed part 
acquires as bright a green as the leaves. In other Endogens, 
the leaf-sheaths of which are successively burst and exfo- 
liated by the swelling axis, it may be observed that where 
the dead sheaths do not much obstruct the light and air, 
the surface of the axis underneath is full of chlorophyll. 
Dendrobium is an example. But when the dead sheaths 
accumulate into an opaque envelope, the chlorophyll dis- 
appears, and also, we may infer, the function its presence 
habitually implies. Carrying with us this evidence, we shall 
recognize a like relation in Exogens. While its outer layer 
remains tolerably transparent, an exogenous stem or branch 
continues to show, by the formation of chlorophyll, that it 
shares in the duties of the leaves; but in proportion as a 
bark which the light cannot penetrate is produced by the 
adherent flakes of dead skin, or by the actual deposit of a 
protective substance, the differentiation of duties becomes 
more decided. Cactuses and Euphorbias supply 


THE OUTER TISSUES OF PLANTS. 241 


us with converse facts having the same implication. The 
swollen succulent axes so strangely combined in these plants, 
maintain for a long time the transparency of their outer 
layers; and doing this, they so efficiently perform the offices 
of leaves that leaves are not produced. In some cases, axes 
that are not succulent participate largely in the leaf-func- 
tion, or entirely usurp it—still, however, by fulfilling the 
same essential conditions. Occasionally, as in Statice bras- 
sicepha, stems become fringed; and the fringes they bear 
assume, alone with the thinness of leaves, their darker green 
and general aspect. In the genus Ruscus, the flattened axis 
simulates so closely the leaf-structure, that were it not for the 
flower borne on its midrib, or edge, its axial nature would 
hardly be suspected. And let us not omit to note that where 
axes usurp the characters of leaves, in their attitudes as well 
as in their shapes and thickness, there exist contrasts between 
their under and upper surfaces, answering to the contrasts 
between the relations of these surfaces to the light. Of this 
Ruscus androgynus furnishes a striking example. In it the 
difference which the unaided eye perceives is much less con- 
spicuous than that disclosed by the microscope; for I find 
that while the face of the pseudo-leaf has no stomata, the back 
is abundantly supplied with them. One wore illustration must 
be added. Equally for the morphological and physiological 
truths which it enforces, the Coccoleba platycladon is one of 
the most instructive of plants. In it the simulation of forms 
and usurpation of functions, are carried out in a much more 
marvellous way than among the Cactacee. Imagine a growth 
resembling in outline a very long willow-leaf, but without a 
midrib, and having its two surfaces alike. Imagine that 
across this thin, green, semi-transparent structure, there are 
from ten to thirty divisions, which prove to be the successive 
nodes of an axis. Imagine that along the edges of this 
jeaf-shaped aggregate of internodes, there arise axillary 
buds, some of which unfold into flowers, and others of which 


shoot up vertically into growths like the one which bears 
VOL. I. 16 


242 PHYSIOLOGICAL DEVELOPMENT. 


them. Imagine a whole plant thus seemingly composed of 
jointed willow-leaves growing from one another’s edges, 
and some conception will be formed of the Coccoloba. The 
two facts which have meaning for us here are—first, that the 
performance of leaf-functions by these axes goes along with 
the assumption of a leaf-like translucency ;. and, second, 
that these flattened axes, retaining their upright attitudes, 
and therefore keeping their two faces similarly conditioned, 
have these two faces alike in colour and texture. 

That physiological differentiation of the surface which 
arises in Phanogams between axial organs and foliar organs, 
is thus traceable with tolerable clearness to those differences 
between their conditions which integration has entailed— 
partly in the way above described, and partly in other ways 
still to be named. By its relative position, as being shaded 
by the leaves, the axis is less-favourably circumstanced for 
performing those assimilative actions effected by the aid of 
light. Further, that relatively-small ratio of surface to 
mass in the axis, which is necessitated by its functions asa 
support and a channel for circulation, prevents it from taking 
in, with the same facility as the leaves, those surrounding 
gases from which matter is to be assimilated. Both these 
special causes, however, in common with that previously 
assigned, fall within the general cause. And in the fact that 
where the differential conditions do not exist, the physio- 
logical differentiation does not arise, or is obliterated, we 
have clear proof that it is determined by unlikenesses in the 
relations of the parts to the environment. 


§ 274. From this most general contrast between aérial 
surface-tissues—those of axes and those of folia—we pass now 
to the more special contrasts of like kind existing in folia 
themselves. Leaves present us with superficial differentia- 
tions of structure and function ; and we have to consider the 
relations between these and the environing forces. 

Over the whole surface of every chee amic leaf, as over 


THE OUTER TISSUES OF PLANTS. 243 


the fronds of the higher Acrogens, there extends a simple or 
compound cuticular layer, formed of cells that are closely 
united at their edges and devoid of that granular colouring 
matter contained in the layers of cells they inclose: the 
result being that the membrane formed of them is compara- 
tively transparent. On the submerged leaves of aquatic 
Pheenogams, this outer layer is thin, delicate, and permeable 
-by water; but on leaves exposed to the air, and especialiy 
on their upper surfaces, it is comparatively strong, dense, 
often smooth, and impermeable by water: being thus fitted 
to prevent the rapid escape of the contained juices by evapo- 
ration. Similarly, while the leaves of terrestrial plants that live 
in temperate climates, usually have comparatively thin coats 
thus composed, in climates that are both hot and dry, leaves 
are commonly clothed with two, three, or more layers of such 
cells. Nor is thisall. The outside of an aérial leaf differs from 
that of a submerged leaf by containing a deposit of waxy sub- 
stance. Whether this be exuded by the exposed surfaces 
of the cells, as some contend, or whether it is deposited within 
the cells, as thought by others, matters not in so far as the 
general result is concerned. In either case a waterproot 
coating is formed at the outermost sides of these outermost 
cells; and in many cases produces that polish by which the 
upper surface of the leaf is more or less distinguished from 
the under surface. This external pellicle pre- 
sents us with another contrast of allied meaning. On the 
upper surfaces of leaves subject to the direct action of the 
sun’s rays, there are either few or none of those minute 
openings, or stomata, through which gases can enter or 
escape; but on the under surfaces these stomata are abun- 
dant—a distribution which, while permitting free absorp- 
tion of the needful carbonic acid, puts a check on the 
exit of watery vapour. Two general exceptions to this ar- 
rangement may be noted. Leaves that float on the water 
have all their stomata on their upper sides, and leaves that are 


submerged have no stomata—modifications obviously ap- 
16 * 


244 PHYSIOLOGICAL DEVELOPMENT. 


propriate to the conditions. What is to be 
said respecting the genesis of these differentiations? For 
the last there seems no direct cause: its cause must be in- 
direct. The unlike actions to which the upper and under 
surfaces of leaves are subject, have no apparent tendency to 
produce unlikeness in the number of their breathing holes. 
Here the natural selection of spontaneous variations furnishes 
the only feasible explanation. Jor the first, however, there 
is a possible cause in the immediate actions of incident 
forces, which survival of the fittest continually furthers. The 
substance contained in the cells of leaves consists partly 
of wax and partly of chlorophyll. According to Mulder, 
“there is a genetic connexion between the production, of wax 
and that of the green colouring matter in the leaves ;” and 
he alleges, as the result of his own experiments and those of 
Berzelius, that chlorophyll “may be decomposed, both by 
oxidizing and de-oxidizing substances, so as to become colour- 
less at last; and that wax seems to be producible from it by 
de-oxidizing actions.” Now the superficial cells of leaves 
are more exposed to the de-oxidizing influence of light than 
the inner cells; those forming the upper surface are more 
exposed to it than those forming the under surface; and 
those which coat leaves in hot dry climates are more exposed 
to it than those by which leaves in temperate climates are 
coated. May it not be that the action of light, whence 
chlorophyll results as a transitional compound which after- 
wards passes into a colourless compound, is an action directly 
tending to form these bleached and transparent outer layers ; 
and directly tending to produce a greater thickness of such 
layers in proportion as itis intense? There are difficulties 
in the way of this supposition ; for I learn from Dr. Hooker 
that some of the Balanophore, which grow in the shade, are 
very full of wax. As these are parasites, however, and absorb 
the prepared juices of other plants, the comparison is interfered 
with. But whatever be its origin, we have to note that this 
waxy substance suspended in the fluid which these bleached 


THE OUTER TISSUES OF PLANTS. 245 


outer cells contain, must be deposited as fast as the fluid 
escapes. Where will it be deposited? The fluid exhaling 
through the walis of the cells next the air, will be likely to 
leave behind the suspended substance attached to these walls. 
On remembering the pellicle that is apt to form on thick 
solutions or emulsions as they dry, and how this pellicle as it 
grows retards the further drying, it will be perceived that 
the deposit of waxy substance next to the outer surfaces of 
the cuticular cells in leaves, is probably initiated by the 
evaporation which it eventually checks. We have here, 
indeed, a very simple case of equilibration. Where the loss 
of water is too great, this waxy pellicle left behind by the 
escaping water will protect most those individuals of the 
species in which it is thickest or densest ; and by inheritence 
and continual survival of the fittest, there will be established 
in the species that thickness of the layer which brings the 
evaporation to a balance with the supply of water. 

Another superficial differentiation, still more familiar, has 
to be noted. very child soon learns to distinguish by its 
colour the upper side of a leaf from its under side, if the leaf 
is one that has grown in such way as to establish the rela- 
tions of upper and under. The upper surfaces of leaves are 
habitually of a deeper green than the under. Microscopic 
examination shows that this deeper green results from the 
closer clustering of those parenchyma-cells full of chlorophyll 
that are in some way concerned with the assimilative actions; 
while beneath them are more numerous intercellular passages 
communicating with those openings or stomata through which 
is absorbed the needful air. Now when it is remembered that 
the formation of chlorophyll is clearly traceable to the action 
of light—when it is remembered that leaves are pale where 
they are much shaded and colourless when developed in the 
dark, as in the heart of a Cabbage—when it is remembered 
that succulent axes and petioles, like those of Sea-kale and 
Celery, remain white while the light is kept from them and 
become green when exposed; it cannot be questioned that 


246 PHYSIOLOGICAL DEVELOPMENT. 


this greater production of chlorophyll next to the upper 
surface of a leaf, is directly consequent on the greater 
amount of light received. Here, as in so many other cases, 
we must regard the differentiation as in part due to direct 
equilibration and in part to indirect equilibration. Fa- 
miliar facts compel us to conclude that from the beginning, 
each individual foliar organ has undergone a certain im- 
mediate adaptation of its surfaces to the incidence of light ; 
that when there arose a mode of growth which exposed the 
leaves of successive generations in similar ways, this im- 
mediately-produced adaptation, ever tending to be transmitted, 
was furthered by the survival of individuals inheriting it in 
the greatest degree; and that so there was gradually esta- 
blished that difference between the two surfaces which each 
leaf displays before it unfolds to the light, but which becomes 
more marked when it has unfolded.* 

From the ordinary cases let us now pass to the exceptional 
cases. We will look first at those in which the two faces of 
the leaves differ but little, or not at all—their circumstances 
being similar or equal. Leaves that grow in approximately- 
upright attitudes, and attitudes which do not maintain the 
relative positions of the two surfaces with constancy, may be 
expected to display an unusual likeness between the two 
surfaces; and among them we see it. The Grasses may be 
named as a group exemplifying this relation; and if, instead 
of comparing them as a group with other groups, we compare 


* The current doctrine that chlorophyll is the special substance concerned 
in vegetal assimilation, either as an agent or as an incidental product, must 
be taken with considerable qualification. Besides the fact that among the 
Alga there are many red and black kinds which thrive ; and besides the fact 
that among the lower Acrogens there are species which are purple or chocolate- 
coloured ; there is the fact that Phenogams are not all green. We have the 
Copper-Beech, we have the black-purple Coleus Verschaffeltit, and we have the 
red variety of Cabbage, which seems to flourish as well as the other varieties. 
Chlorophyll, then, must be regarded simply as the most general of the colour- 
ing matters found in those parts of plants in which assimilation is being effected 
by the agency of light. 


THE OUTER TISSUES OF PLANTs. 247 


those dwarf kinds of them which spread out their leaves 
horizontally, with the large aspiring kinds, such as Arundo, 
we trace a like antithesis: in the one the contrast of upper 
and under is very obvious, while in the other it is scarcely to 
be detected. Leaves of various other Endogens that grow 
in a similar way, similarly show us a near approach to uni- 
formity of the two surfaces; as instance the genus Clivia, 
and the thinner-leaved kinds of Yucca. Where the con- 
trast of upper and under is greatly diminished by the as- 
sumption of a rounded or cylindrical form instead of a flat- 
tened form, the same thing happens. The genus Kleinia 
furnishes illustrations. It may be remarked, too, that 
even within the limits of this genus there are instructive 
variations; for while in Kleinia ficoides the leaves, shaped 
like pea-pods, are broadest in a vertical direction, and have 
their lateral surfaces alike in conditions and structure, in 
other species the leaves, broader horizontally than vertically, 
exhibit unlikeness between the upper and under sides. 
Egually to the point is the evidence furnished by vertically- 
growing leaves that are cylindrical, as those of Sanseviera 
cylindrica, or as those of the Rush-tribe: the similarly-placed 
surface has all around a similar character. Of 
kindred meaning, and still more conclusive, are the cases in 
which the under side of the leaf, being more exposed to 
light than the upper side, usurps the character and function 
of the upper side. If a common Flag be pulled to pieces, 
it will be seen that what answers to the face in other 
leaves, forms merely the inside of the sheath including the 
younger leaves, and is obliterated higher up. ‘The two sur- 
faces of the blade answer to the two under halves of a 
leaf that has been, as it were, folded together lengthways, 
with the two halves of its upper surface in contact. And 
here, in default of an upper surface, the under surface acquires 
its character and discharges its function. A like substitution 
occurs in Witsenia corymbosa; and there are some of the 
Orchids, as Lockhartia, which display it in a very obvious way. 


248 PHYSIOLOGICAL DEVELOPMENT. 


When joined with the foregoing evidence, the evidence 
which another kind of substitution supplies is of great 
weight. I refer to that which occurs in the Australian 
Acacias, already instanced as throwing light on morpho- 
logical changes. In these plants the leaves properly so called 
are undeveloped, and the footstalks, flattened out into folia- 
ceous shapes, acquire veins and midribs, and so far simulate 
leaves as ordinarily to be taken for them—a fact in itself of 
much physiological significance. But that which it concerns 
us especially to note, is the absence of distinction between 
the two faces of these phyllodes, as they are named, and the 
cause of its absence. These transformed petioles do not 
flatten themselves out horizontally, so as to acquire under 
and upper sides, as most true leaves do; but they flatten 
themselves out vertically: the result being that their two 
sides are similarly circumstanced with respect to light and 
other agencies; and there is consequently nothing to cause 
their differentiation. And then we find an analogous case 
where differential conditions arise, and where some differen- 
tiation results. In Ovalis bupleurifolia, Fig. 66, there is a 
similar flattening out of the petiole into a pseudo-leaf; but 
in it the flattening takes place in the same plane as the leaf, 
so as to produce an under and an upper surface; and here 
the two surfaces of the pseudo-leaf are slightly unlike—in 
contour if in nothing else. 


§ 275. We come now to such physiological differentiations 
among the outer tissues of plants, as are displayed in the © 
contrasts between foliar organs on the same axis, or on 
different axes—contrasts between the seed-leaves and the. 
leaves subsequently formed, between submerged and aérial 
leaves in certain aquatic plants, between leaves and bracts, 
and between bracts and sepals. To deal even briefly with 
these implies information which even a professed botanist 
would have to increase by special inquiries, before attempting 
interpretations. Here it must suffice to say something 


THE OUTER TISSUES OF PLANTS. 249 


respecting those marked unlikenesses that exist between the 
tissues of the more characteristic parts of flowers, and the 
tissues of the homologous foliar organs. 

It was pointed out in § 196, that the terminal parts of a 
pheenogamic axis have sundry characters in common with such 
fronds as those out of which we concluded that the phzno- 
gaumic axis has arisen by integration—common characters of 
a kind to be expected. In their simple cellular composition, 
comparative want of chlorophyll, and deficiency of vascular 
structures, the undeveloped ends of leaf-shoots and the 
developed ends of flower-shoots, approach to the fronds of the 
simpler Acrogens. We also noted between them another 
resemblance. It is said of the Jungermanniacee, that 
“though under certain circumstances of a pure green, they 
are inclined to be shaded with red, purple, chocolate, or other 
tints ;” and answering to this we have the facts that such 
colours commonly occur in the terminal folia of a pheeno- 
gamic axis when arrest of its development leads to the 
formation of a flower, and that very frequently they are 
visible at the ends of leaf-axes. In the unfolding parts of 
shoots, more or less of red, or copper-colour, or chocolate- 
colour, may generally be seen: often indeed it charac- 
terizes the leaves for some time after they are unfolded. 
Occasionally the traces of it are permanent; and, as in 
the scarlet terminal leaves of Poinsettia pulcherrima, we see 
that it may become, and continue, extremely conspicuous. 
The question, then, now to be asked, is—has this colouring 
by which the immature part of the phenogamic axis is cha- 
racterized, anything to do with the colouring of flowers ? 
Has this difference between undeveloped folia and folia that 
are further developed, been increased by natural selection 
where an advantage accrued from it, until it has ended in 
the strong contrast we now see? I think we may not irra- 
tionally infer that this has been the case. 

Facts, very numerous and varied, united to warrant us in 
concluding that gamogenesis commences where the forces 


250 PHYSIOLOGICAL DEVELOPMENT 


that conduce to growth are nearly equilibrated by the forces 
that resist growth (§ 78); and the induction that in plants, fer- 
tilized germs are produced at places where there is an approach 
towards this balance, we found to be in harmony with the 
deduction that an advantage to the species must be gained 
by sending off migrating progeny from points where nutri- 
tion is failing. Other things equal, failure of nutrition 
may be expected in parts that have the most remote or most 
indirect access to the materials furnished by the roots— 
materials that have to be carried great distances by a very 
imperfect apparatus. The ends of lateral axes are therefore 
the probable points of fructification, in aggregates of the 
third order that have taken to growing vertically. But 
if these points at which nutrition is failing, are also the 
points at which the colours inherited from lower types are 
likely to recur in more marked degrees than elsewhere ; then 
we may infer that the organs of fructification will not un- 
frequently co-exist with such colours at the ends of such 
axes. How may the resulting contrast between the older 
fronds and the fronds next the germ-producing organs be 
increased? If uninterfered with it would be likely to di- 
minish. These traits inherited from remote ancestry, might 
be expected slowly to fade away. How, then, is the intensi- 
fication of them to be explained ? 

If a contrast of the kind described favours the propagation of 
a race in which it exists, it will be maintained and increased ; 
and if we take into account an agency of which Mr. Darwin has 
shown the great importance—the agency of insects—we shall 
have little difficulty in understanding how such a contrast 
may facilitate propagation. We cannot, of course, here assume 
the agency of insects so specialized in their habits as Bees and 
Butterflies; for their specialized habits imply the pre-exist- 
ence of the contrast to be explained. But there is an insect- 
agency of a more general kind which may be fairly counted 
upon as coming into action. Various small Flies and 
Beetles wander over the surfaces of plants in search of 


THE OUTER TISSUES OF PLANTS. BOL 


food. It is.a legitimate assumption that they will frequent 
most those parts in which they find most food, or food most 
to their liking—especially if at the same time they gain the 
advantage of concealment. Now the ends of axes, formed of 
young, soft, and closely-packed folia, are the parts which more 
than any others offer these several advantages. ‘They afford 
shelter from enemies ; they frequently contain exuded juices ; 
and when they do not, their tissues are so tender as to be 
easily pierced in search of the sap. If, then, from the first, as 
at presert, these ends of axes have been favourite haunts of 
small insects; and if, where the closely-clustered folia con- 
tained the generative organs, the insects frequenting them 
occasionally carried adherent fructifying cells from one plant 
to another, and so aided fertilization; it would follow that 
anything which made such terminal clusters more attractive 
to such insects, or more conspicuous to them, or both, would 
further the multiplication of the race, and would so be con- 
tinually increased by the extra multiplication of individuals 
in which it was greatest. Here we find the clue. ‘This con- 
trast of colour between the folia next to the fructifying parts 
and all other folia, must constantly have facilitated insect- 
agency; supposing the insects to have had the power of dis- 
tinguishing between colours. That Bees and Butterflies 
have this power is manifest: they may be watched fly- 
ing from flower to flower, disregarding all other parts 
of the plants. And if the less-specialized insects pos- 
sessed some degree of such discrimination, then the initial 
contrasts of colour above described would be maintained. 
‘and increased. Let such a connexion be once established, and 
it must tend to become more decided. Insects most able 
to discern the parts of plants which afford what they seek, 
will be those most likely to survive and leave offspring. 
Plants presenting most of the desired food, and showing most 
clearly where it lies, will have their fertilization and multi- 
plication furthered in the greatest degree. And so the 
mutual adaptation will become ever closer; while it is ren- 


Big PHYSIOLOGICAL DEVELOPMENT. 


dered at the same time more varied by the special require- 
ments of the insects and of the plants in each locality, under 
each change of conditions. Of course, the 
genesis of the sweet secretions and the odours of flowers, 
has a parallel interpretation. The simultaneous production 
of honey, or some kindred substance, is implied above; 
since, unless a bait co-existed with the colour, the colour 
would not attract insects, and would not be maintained 
and intensified by natural selection. Gums, and resins, 
and balsams, are familiar products of plants; apparently, 
in many cases, excreted as useless matters from various 
parts of their surfaces. These substances, admitting of 
wide variations in quality, as they do, afford opportunities 
for the action of natural selection wherever any of them 
attractive to insects, happen to be produced near the organs 
of fructification. And this action of natural selection once 
set up, may lead to the establishment of a local excretion, to 
the production of an excretion more and more attractive, and 
to the disposal of the organ containing it in such a way as 
most to facilitate the carrying away of pollen. Similarly 
and simultaneously with odours. Odours, like colours, draw 
insects to flowers. After observing how Bees come swarming 
into a house where honey is largely exposed, or how Wasps 
find their way into a shop containing much ripe fruit, it 
cannot be questioned that insects are to a considerable extent 
guided by scent. Being thus sensitive to the aromatic sub- 
stances which flowers exhale, they may, when the flowers are 
in large masses, be attracted by them from distances at which 
the flowers themselves are invisible. And manifestly, the 
flowers which so attract them from the greatest distances, 
increasing thereby their chances of efficient fertilization, will 
be most likely to perpetuate themselves, That is to say, 
survival of the fittest must tend to produce perfumes that 
are both more powerful and more attractive. 

These physiological differentiations, then, which mark off 
the foliar organs of flowers from other foliar organs, are 


THE OUTER TISSUES OF PLANTS. 203 


the consequences of indirect equilibration. They are not 
due to the immediate actions of unlike incident forces on 
the parts of the individual plant; but they are due to the 
actions of such unlike incident forces on the aggregate of 
individuals, generation after generation.* 


§ 276. The unity of interpretation which we here find for 
phenomena of such various orders, could hardly be found 
were the phenomena otherwise caused. That the stronger 
and the feebler contrasts among the different parts of the 
outer tissues in plants, should so constantly occur along with 
stronger and feebler contrasts among the incident forces, is 
in itself weighty evidence that unlike outer actions have 
-caused unlike inner actions, and correspondingly-unlike struc- 
tures; either by changing the functional equilibrium in the 
individual, or by changing it in the race, or by both. 

Even in the absence of more direct proof, there would be 
-great significance in the marked differences that habitually 
-exist between the exposed and imbedded parts of plants, 
between the stems and the leaves, and between the upper and 
~under surfaces of the leaves. The significance of these diffo- 
-rences is increased when we discover that they vary in degree 


* This seems as fit a place as any for noting the fact, that the greater part 
-of what we call beauty in the organic world, is in some way dependent on 
the sexual relation. It is not only so with the colours and odours of flowers. 
It is so, too, with the brilliant plumage of birds, and with the songs of birds, 
both of which, in Mr. Darwin’s view, are due to sexual selection ; and it is 
probable that the colours of the more conspicuous insects are in part similarly 
determined. The remarkable circumstance is, that these characteristics, which 
have originated by furthering the production of the best offspring, while they 
are naturally those which render the organisms possessing them attractive to 
one another, directly or indirectly, should also be those which are so generally 
attractive to us—those without which the fields and woods would lose hait 
their charm. It is interesting, too, to observe how the conception of human 
beauty is in a considerable degree thus originated. And the trite obser- 
vation that the element of beauty which grows out of the sexual relation 
is so predominant in esthetic products—in music, in the drama, in fiction, in 
“poetry—gains a new meaning when we see how deep down in organic nature 
this connexion extends. 


254 PHYSIOLOGICAL DEVELOPMENT. 


as the differences in the conditionsvaryin degree. Still greater 
becomes the force of the evidence on finding that these 
strongly-contrasted parts may, when placed in one another's 
conditions, and kept in them from generation to generation, 
permanently assume one another's functions, and, in a great 
degree, one another's structures. Hven more conclusive 
yet is the argument rendered, by the discovery that, where 
these substitutions of function and structure take place, the 
superinduced modifications differ in different circumstances ; 
just as the original modifications do. The fact that a flattened 
stem simulating a vertically-growing leaf has its two surfaces 
alike, while when it simulates a horizontally-growing leaf its 
upper and under surfaces differ, is a fact which, standing 
alone, might prove little, but proves much when joined with 
all the other evidence. And its profound meaning becomes 
the more obvious on discovering that the same thing happens 
- with petioles when they usurp leaf-functions. 

Finally, when we remember how rapidly analogous modi- 
fications of function and structure arise in, the superficial 
tissues of individual plants, the general inference can scarcely 
be resisted. When we meet with so striking a case as that 
of the Begonia-leaf, a fragment of which stuck in the ground 
produces roots from its under surface and leaves from its 
upper surface—when we see that though, in this case, the 
typical structure of the plant presently begins to control the 
organizing process, yet the initial differentiations are set up 
by the differential actions of the environment; the presump- 
tion becomes extremely strong that the heterogeneities of 
surface which we have considered, result, as alleged, directly 
or indirectly from heterogeneities in the incident forces. 


CHAPTER IY. 
DIFFERENTIATIONS AMONG THE INNER TISSUES OF PLANTS.* 


§ 277. In passing from plants formed of threads or thin 
laminee, to plants having some massiveness, we find that after 
the external and internal parts have become distinguished from 
one another, there arise dictinctions among the internal parts 
themselves, as well as among the external parts themselves: 
the primarily-differentiated parts are both re-differentiated. 

From types of very low organisation illustrations of this 
may be drawn. In the thinner kinds of Laminaria there 
exists but the single contrast between the outer layer of cells 
and an inner layer; but in larger species of the same genus, 
as L. digitata, there are three unlike layers on each side of a 
central layer differing from them—augmentation of bulk is 
accompanied by multiplication of concentric internal struc- 
tures, having their unlikenesses obviously related to unlike- 
nesses in their conditions. In Furcellaria and various Alge 
of similarly swollen forms, the like relation may be traced. 

Just indicating the generality of this contrast, but not 


* Students of vegetal physiology, familiar with the controversies respecting 
sundry points dealt with in this chapter, will probably be surprised to find 
taken for granted in it, propositions which they have habitually regarded as 
open to doubt. Hence it seems needful to say that the conclusions here set 
forth, have resulted from investigations undertaken for the purpose of forming 
ppinions on several unsettled questions which I had to treat, but which I 
could find in books no adequate data for treating. The details of these inves- 
tigations, and the entire argument of which this chapter is partly an abstract, 
will be found in Appendix C, 


256 PHYSIOLOGICAL DEVELOPMENT. 


attempting to seek in these lower types for any more specific 
interpretation of it, let us pass to the higher types. The 
argument will be amply enforced by the evidence obtained 
from them. We will look first at the conditions which they 
have to fulfil; and then at the way in which the functions 
and structures adapting them to these conditions arise. 


§ 278. A terrestrial plant that grows vertically needs no 
marked modification of its internal tissues, so long asthe height 
it reaches is very small. As we before saw, the spiral or 
cylindrical rolling up of a simple cellular frond, or the more 
bulky growth of a simple cellular axis, may give the requisite 
strength; and the requisite circulation may be carried on 
through the unchanged cellular tissue. But in proportion 
as the height to be attained and the mass to be supported 
increase, the supporting part must acquire greater bulk or 
greater density, or both; and some modification that shall 
facilitate the transfer of nutritive liquids must take place. 
Hence, in the inner tissues of plants we may expect to find 
that structural changes answering to these requirements 
become marked, as the growth of the aérial part becomes 
great. Facts correspond with these expectations. 

Among the humbler Acrogens, which creep over, or raise 
themselves but little above, the surfaces they flourish upon, 
there is scarcely any internal differentiation: the vascular 
and woody structures, if not in all cases absolutely un- 
represented, are rarely and very feebly indicated. But 
among the higher Acrogens—the Ferns and Lycopodiums— 
which raise their fronds to considerable heights, there are 
vascular bundles and hard tissues like wood; and by the 
Tree-Ferns massive axes are developed. That the relation 
which thus shows itself among Cryptogams is habitual among 
Phzenogams, scarcely needs saying. 

Pheenogams, however, are not universally thus charac- 
terized in a decided way. Besides the comparative want of 
woody substance in flowering plants of humble growth, and 


THE INNER TISSUES OF PLANTs. 237 


besides the paucity of vessels in ordinary water-plants, there 
are cases of much more marked divergence from this typical 
internal structure. These exceptional cases occur under 
exceptional conditions, and are highly instructive. They 
are of two kinds. One group of them is furnished 
by certain plants that are parasitic on the exposed roots of 
trees—parasitic not partially,. as the Mistletoe, but to the 
extent of subsisting wholly on the sap they absorb. Fungus- 
hike in colour and texture, and having scales for leaves, these 
Balanophore and Rafflesiacee are recognizable as Phenogams 
by scarcely any other traits than their fructifications. Along 
with their abortive leaves and absence of chlorophyll, there 
is a great degradation of those internal tissues by which 
Phenogams are commonly distinguished. Though Dr. 
Hooker has shown that they are not, as some botanists thought, 
devoid of spiral vessels; yet, as shown by the mistake 
previously made in classifying them, their appliances for 
circulation are rudimentary. And this trait goes along with 
a greatly-simplified distribution of nutriment. In the 
absence of leaves there can be but little down-current of 
nutriment, such as leaves usually supply to roots: there 
cannot be much beyond an upward current of the absorbed 
juices. The other cases occur where circulation 
is arrested or checked in a different way; namely, in 
plants that are wholly submerged. These are the Podo- 
stemones, which are aquatic even to the extent of flowering 
under water. Clothing as they do the submerged rocks 
in tropical rivers, their roots, like those of the Alga, serve 
only for attachment; their foliar expansions, frond-like in 
shape, are everywhere bathed by the water; and their organs 
of fructification never exposed to the air, but perhaps aided 
in their functions by water-insects instead of air-insects, are 
the only marked signs of kinship to other Pheenogams. Observe 
then the connexion of facts. One of these Podestemones needs 
no internal stiffening substance, for it exists in a medium of 


its own specific gravity ; and having no unlikeness between 
VOL. IL 7 


258 PHYSIOLOGICAL DEVELOPMENT. 


the materials assimilated at its fixed and its free ends, it has 
no need for a circulation—nor, indeed, in the absence of 
evaporation from any part of its surface, could any active 
circulation take place. Here, accordingly, the ordinary 
internal structures are undeveloped: though spiral vessels 
are not entirely absent, yet they are so rare as to do no more 
than verify the inference of phzenogamic relationship drawn 
from the flowers. 

The method of agreement, the method of difference, and 
the method of concomitant variations, thus unite in proving 
a direct relation between the demand for support and cir- 
culation, and the existence of these vascular woody bundles 
which the higher plants habitually possess. The question 
which we have to consider is—Under what influences are 
these structures, answering to these requirements, developed ? 
How are these internal differentiations caused ? The inquiry 
may be conveniently divided. Though the supporting tissues 
and the tissues concerned in the circulation of liquids are 
closely connected, and indeed entangled, with one another, 
we may fitly deal with them apart. Let us take first the 
supporting tissue. 


§ 279. Many common-place facts indicate that the me- 
chanical strains to which upright-growing plants are exposed, 
themselves cause increase of the dense deposits by which such 
plants are enabled to resist such strains. There is the fact 
that the massiveness of a tree-trunk varies according to the 
stress habitually put upon it. If the contrast between the 
slender stem of a tree growing in a wood and the bulky stem 
of a kindred tree growing in the fields, be ascribed to differ- 
ence of nutrition rather than difference of exposure to winds: 
there is still the fact that a tree trained against a wall has a 
less bulky stem than a tree of the same kind growing un- 
supported ; and that between the long weak branches of the 
one and the stiff ones of the other there are decided contrasts. 
Tf it be objected that a tree so trained and branches so borne 


THE INNER TISSUES OF PLANTS. 259 


have relatively less foliage, and that therefore these unlike- 
nesses also are due to unlikenesses of general nutrition, which 
may in part be true; there are still such cases as those of 
garden plants, which when held up by tying them to sticks 
have weaker stems than when they are unpropped, and sink 
down if their props are taken away. Again, there is the 
evidence supplied by roots. Though the contrast between 
the feeble roots of a sheltered tree and the strong roots of 
an exposed tree, may, like the contrast of their stems, be 
mainly due to difference of nutrition, and therefore supplies 
but doubtful evidence, we get tolerably clear evidence where 
trees growing on inclined rocky surfaces, send into crevices 
that afford little moisture or nutriment, roots which never- 
theless become thick where they are so directed as to bear 
great strains. Suspicion thus raised is strengthened 
into conviction by special evidences occurring in the places 
where they are to be expected. The Cactuses, with their 
succulent growths that pass into woody growths slowly and 
irregularly, give us the opportunity of tracing the conditions 
under which the wood is formed. Good examples occur in the 
genus Cereus, and especially in forms like C. crenudatus. Here, 
from a massive vertically-growing rod of fleshy tissue, two 
inches or more in diameter, there grow at intervals lateral rods 
similarly bulky, which, quickly curving themselves, take 
vertical directions. One of these heavy branches puts great 
strains on its own substance and that of the stem at their 
point of junction’; and here both of them become brown and 
hard, while they continue green and succulent all around. 
Such diiferentiations may be traced internally before they 
are visible on the surface. If a joint of an Opuntia be sliced 
through longitudinally, the greater resistance to the knife 
all around the narrow neck, indicates there a larger deposit 
of lignin than elsewhere ; and a section of the tissue placed 
under the microscope, exhibits at the narrowest part a con- 
€entration of the woody and vascular bundles. Clear 


evidence of another kind has been noted by Mr. Darwin, in the 
17 * 


260 PHYSIOLOGICAL DEVELOPMENT. 


organs of attachment of climbing plants. Speaking of Solanum 
jusminoides he says:—‘ When the flexible petiole of half- 
or a quarter-grown leaf has clasped any object, in three or 
four days it increases much in thickness, and after several 
weeks becomes wonderfully hard and rigid; so that I could 
hardly remove one from its support. On comparing a thin 
transverse slice of this petiole with one from the next or 
older leaf beneath, which had not clasped anything, its 
diameter was found to be fully doubled, and its structure 
greatly changed. * * * This clasped petiole had actually 
become thicker than the stem close beneath; and this was 
chiefly due to the greater thickness of the ring of wood, 
which presented, both in transverse and longitudinal sections, 
a closely similar structure in the petiole and axis. The 
assumption by a petiole of this structure is a singular 
morphological fact; but it is a still more singular physio- 
logical fact that so great a change should have been induced 
by the mere act of clasping a support.” 

If there is a direct relation between mechanical stress and 
the formation of wood, it ought to explain for us the internal 
distribution of the wood. Let us see whether it does this. 

When seeking in mechanical actions and reactions the 
cause of that indurated structure which forms the verte- 
brate axis (§§ 254-7), 1t was pointed out that in a transversely- 
strained mass, the greatest pressures and tensions are thrown 
on the molecules of the concave and convex surfaces. Hence, 
supposing the transversely-strained mass to be a cylinder, 
bent backwards and forwards not in one plane but now in 
this plane and now in that, its peripheral layers will be 
those on which the greatest stress falls. An ordinary 
exogenous axis is such a cylinder so strained. The main- 
tenance of its attitude either as a lateral shoot or a vertical 
shoot, implies subjection to the bendings caused by its own 
weight and by the ever-varying wind. These bendings 
imply tensions and pressures falling most severely first on 
one side of its outer layers and then on another. And if the 


THE INNER TISSUES OF PLANTS. 261 


dense substance able to resist these tensions and pressures is 
deposited most where they are greatest, we ought to find it 
taking the shape of a cylindrical casing. This is just what 
we do find. On cutting across a shoot in course of formation, 
we see its central space either unoccupied or occupied only 
by soft tissue. That the layer of hard tissue surrounding 
this is not the outermost layer, is true: there lies beyond it 
the cambium layer, from which it is formed. But outside 
of the cambium there is another layer of dense tissue, the 
liber, having frequently a tenacity greater even than that, of 
the wood—a layer which, while it protects the cambium and 
offers additional resistance to the transverse strain, admits of 
being fissured as fast as the cylinder of wood thickens. That 
is to say, the deposit of resisting substance is as completely 
peripheral as the exogenous mode of growth permits. So, 
too, in general arrangement is it with the endogenous stem. 
Different as is here the mode of growth, and different as is 
_the internal structure, there yet holds the same general dis- 
tribution of tissues, answering to the same mechanical con- 
ditions. The vascular woody bundles, more abundant towards 
the outside of the stem than near the centre, produce a harder 
casing surrounding a softer core. In the supporting 
structures of leaves we find significant deviations from this 
arrangement. While axes are on the average exposed to 
equal strains on all sides, most leaves, spreading out their 
surfaces horizontally, have their petioles subject to strains 
that are not alike in all directions; and in them the hard 
tissue is differently arranged. Its transverse section is 
not ring-shaped but crescent-shaped: the two horns being 
directed towards the upper surface of the petiole. That this 
arrangement is one which answers to the mechanical con- 
ditions, is not easy to demonstrate: we must satisfy ourselves 
by noting that here, where the distribution of forces is 
different, the distribution of resisting tissue is different. And 
then, showing conclusively the connexion between these differ- 
ences, we have the fact that in petioles growing vertically 


26:2 PHYSIOLOGICAL DEVELOPMENT. 


and supporting peltate leaves—petioles which are therefore 
subject to equal transverse strains on all sides—the vascular 
bundles are arranged cylindrically, as in axes. 

Such, then, are some of the reasons for concluding that the 
development of the supporting tissue in plants, is caused by 
the incident forces which this tissue has to resist. The 
individuals in which this direct balancing of inner and outer — 
actions progresses most favourably, are those which, other 
things equal, are most likely to prosper; and by habitual 
survival of the fittest, there is established a systematic and 
constant distribution of a deposit adapted to the circumstances 
of each type. 


§ 280. The function of circulation may now be dealt with. 
We have to consider here by what structures this is dis- 
charged; and what connexion exists between the demand 
for them and the genesis of them. 

The contrast between the rates at which a dye passes 
through simple cellular tissue and cellular tissue of which the 
units have been elongated, indicates one of the structural 
changes required to facilitate circulation. If placed with its 
cut surface in a coloured liquid, the parenchyma of a potato 
or the medullary mass of a cabbage-stalk, will absorb the 
liquid with extreme slowness; but if the stalk of a fungus be 
similarly placed, the liquid runs up it, and especially up its 
loose central substance, very quickly. On comparing the 
tissues which thus behave so differently, we find that whereas 
in the one case the component cells, packed close together, 
have deviated from their primitive sphericity only as much as 
mutual pressure necessitates, in the other case, they are drawn 
out into long tubules with narrow spaces among them—the 
greatest dimensions of the tubules and the spaces being in the 
direction which the dye takes so rapidly. That which we 
should infer, then, from the laws of capillary action, is 
experimentally shown: liquid moving through tissues follows 
the lines in which the elements of the tissues are most 


THE INNER TISSUES OF PLANTS. 263 


elongated. It does this for two reasons. That narrowing of 
the cells and intercellular spaces which accompanies their 
elongation, facilitates capillarity ; and at the same time fewer 
of the septa formed by the joined ends of the cells have to be 
passed through in a given distance. Hence the 
general fact that the establishment of a rudimentary vascular 
system, is the formation of bundles of cells lengthened in the 
direction which the liquid is to take. This we see very 
obviously among the lower Acrogens. In one of the lichen- 
like Liverworts, the veins which, branching through its 
frond, serve as communications with its scattered rootlets, are 
formed of cells longer than those composing the general tissue 
of the frond: the lengths of these cells corresponding in their 
directions with the lengths of the veins. So, too, is it 
with the midribs of such fronds as assume more definite 
shapes; and so, too, is it with the creeping stems which 
unite many such fronds. That is to say, the current which 
sets towards the growing part from the part which supplies 
the materials for growth, sets through a portion of the tissues 
composed of units that are longer in the line of the current 
than at right angles to that line. The like is true 
of Phenogams. Omitting all other characteristics of those 
parts of them through which chiefly the currents of sap 
flow, we find the uniform fact to be that they consist of cells 
and intercellular spaces distinguished from others by their 
lengths. It is thus with veins, and midribs, and petioles; 
and if we wish proof that it is thus with stems, we have but 
to observe the course taken by a coloured solution into which 
a stem is inserted. 

What is the original cause of this differentiation? Is it 
possible that this modification of cell-structure which favours 
the transfer of liquid towards each place of demand, is itself 
caused by the current which the demand sets up? Does the 
stream make its own channel? ‘There are various reasons 
for thinking that it does. In the first place, the simplest and 
earliest channels, such as we see in the Liverworts, do not 


264 PHYSIOLOGICAL DEVELOPMENT. 


develop in any systematic way, but branch out irregularly, 
following everywhere the irregular lobes of the frond as 
these spread ; and on examining under a magnifier the places 
at which the veins are lost in the cellular tissue, it will be 
seen that the cells are there slightly longer than those 
around: suggesting that the lengthening of them which 
produces an extension of the veins, takes place as fast as 
the growth of the tissue beyond causes a current to pass 
through them. In the second place, a disappearance of the 
granular contents of these cells accompanies their union 
into a vein—a resuit which the transmission of a current 
may not improbably bring about. But be the special causes 
of this differentiation what they may, the evidence favours 
very much the conclusion that the general cause is the 
setting up of a current towards a place where the sap is 
being consumed. In the histological development 
of the higher plants we find confirmation The more 
finished distributing canals in Pheenogams are formed of cells 
previously lengthened. At parts of which the typical struc- 
ture is fixed, and the development direct, this fact is not easy to 
trace ; the cells rapidly take their fibrous structures in antici- 
pation of their pre-determined functions. But in places 
where new vessels are required in adaptation to a modify- 
ing growth, we may clearly trace this succession. The 
swelling root of a turnip, continually having its vascular 
system further developed, and the component vessels 
lengthened as well as multiplied, gives us an opportunity of 
watching the process. In it we see that the reticulated cells 
which unite to form ducts, arise in the midst of bundles of 
ceils that have previously become elongated, and that they 
arise by transformation of such elongated cells; and we 
also see that these bundles of elongated cells have an 
arrangement quite suggestive of their formation by passing 
currents. 

Are there grounds for thinking that these further trans- 
formations by which strings of elongated cells pass into 


THE INNER TISSUES OF PLANTS. 265 


vessels lined with spiral, annular, reticulated, or other 
frameworks, are also in any way determined by the currents 
of sap carried? There are some such grounds. 

As just indicated, the only places where we may look 
for evidence with any rational hope of finding it, are 
places where some local requirement for vessels has arisen 
in consequence of some local development which the type 
does not involve. In these cases we find such evidence. 
Good illustrations occur in those genera of the Cactacee, 
which simulate leaves, like Apiphyllum and Phyllocactus. 
A branch of one of these is outlined in Fig. 256. As before 
explained, this is a flattened axis; and the notches along 
its edges are the seats of the axillary buds. Most of these 
axillary buds are arrested; but occasionally one of them 
grows. Now if, taking an Hpiphylium-shoot which bears 
a lateral shoot, we compare the parts of it that are near 
the abortive axillary buds with the part that is near the 
developed axillary bud, we find a conspicuous difference. 
In the neighbourhood of an abortive axillary bud there 
is no external sign of any internal differentiation; and on 
holding up the branch against the light, the uniform trans- 
lucency shows that there is no greater amount of dense 
tissue near it than in other partr ot the succulent mass. 
But where an axillary bud has developed, a prominent rounded 
ridge joins the midrib of the lateral branch with the midrib 
of the parent branch. In the midst of this rounded ridge 
an opaque core may be seen. And on cutting through it, this 
opaque core proves full of vascular bundles imbedded in 
woody deposits. Clearly, these clusters of vessels imply 
transformations of the tissues, caused by the passage of 
increased currents of sap. The vessels were not there when 
the axillary bud was formed; they would not have de- 
veloped had the axillary bud proved abortive; but they 
arise as fast as growth of the axillary bud draws the sap 
along the lines in which they lie. Verification is obtained 
by examining the internal structures. If longitudinal 


266 PHYSIOLOGICAL DEVELOPMENT. 


sections be made through a growing bud of Opuntia or 
Cercus, it will be found that the vessels in course of for- 
mation converge towards the point of growth, as they would 
do if the sap-currents determined their formation; that 
they are most developed near their place of convergence, 
which they also would be if so produced; and that their 
terminations in the tissue of the parent shoot are partially- 
formed lines of irregular fibrous cells, like those out of 
which the vessels of a leaf or bud are developed. 
Concluding, then, that sap-vessels arise along the lines of 
least resistance, through which currents are drawn or forced, 
the question to be asked is—What physical process produces 
them? Their component cells, united end to end more or less 
irregularly in ways determined by their original positions, 
form a channel much more permeable, both longitudinally 
and laterally, than the tissue around. How is this greater 
permeability caused ? The idea, first propounded 
J believe by Wolff, that the adjoined ends of the cells are 
perforated or destroyed by the passing current, is one for 
which much is to be said. Whether these septa are dissolved 
by the liquids they transmit, or whether they are burst by those 
sudden gushes which, as we shall hereafter see, must frequently 
take place along these canals, needs not be discussed: it is 
sufficient for us that the septa do, in many cases, disappear, 
leaving internal ridges showing their positions ; and, in other 
cases, become extremely porous. Though it is manifest that 
this is not the process of vascular development in tissues that 
unfold after pre-determined types, since, in these, the dehi- 
scences or perforations of septa occur before such direct 
actions can have come into play; yet it is still possible 
that the disappearances of septa which now arise by repe- 
tition of the type were established in the type by such 
direct actions. Be this as it may, however, a 
simultaneous change undergone by these longitudinally- 
united cells must be otherwise caused. Frame-works 2re 
formed in them—frame-works which, closely fitting their inner 


THE INNER TISSUES OF PLANTS. 267 


surfaces, may consist either of successive rings, or continuous 
spiral threads, or networks, or structures between spirals and 
networks, or networks with openings so far diminished that the 
cells containing them are distinguished as fenestrated. Their 
differences omitted, however, these structures have the common 
character that, while supporting the coats of the vessels and 
serving to restore their diameters after they have been com- 
pressed, they also give special facilities for the passage of 
liquids, both through the sides of the transformed cells and 
through their united ends, where these are not destroyed. 
For one of these internal frame-works is not, as usually stated, 
produced by the deposition of substance on the cell-mem- 
brane, in the shape which the frame-work eventually assumes. 
Were it so, this frame-work would have a thickness additional 
to that of the cell-wall as previously existing, which it has not. 
On comparing one of these cells longitudinally cut through, 
with an adjacent cell of the kind to which it was originally 
similar, we see that over every opening in the frame-work, the 
wall of the cell is far thinner than the walls of the adjacent 
cells: the cell-membrane at each of these openings being quite 
bare, instead of being, as in adjacent cells, covered by a layer of 
deposit. Hence this transformation of cells into sap-channels, 
is in part the arrangement or re-arrangement of their sub- 
stance in such ways as greatly to diminish the resistance to 
the passage of liquid, both longitudinally and laterally. 

To attempt any physical interpretation of this change 
is scarcely safe: the conditions are so complex. ‘There are 
many reasons for suspecting, however, that it arises from a 
vacuolation of the substance deposited on the cell wall. It 
rapidly deposited, as it is likely to be along lines where sap 
is freely supplied, this may, in passing from the state of a 
soluble colloid to that of an insoluble colloid, so contract as to 
leave uncovered spaces on the cell-membrane; and this 
change, originally consequent on a physico-chemical action, 
may be so maintained and utilized by natural selection, as to 
result in structures of a definite kind, regularly formed ir 


268 PHYSIOLOGICAL DEVELOPMENT. 


growing parts in anticipation of functions to be afterwards 
discharged. But, without alleging any special cause for this 
metamorphosis, there is good evidence that it is in some way 
consequent upon the carrying of sap. If we examine tissues 
such as that in the interior of a growing turnip that has 
not yet become stringy, we may, in the first place, find 
bundles of elongated cells not having yet deveioped in them 
those fenestrated or reticulated structures by which the ducts 
are eventually characterized. Along the centres of adjacent 
bundles we may find incomplete lines of such cells—some that 
are partially or wholly transformed, with some between them 
that are not transformed. In other bundles, completed chains 
of such transformed cells are visible. And then, in still 
older bundles, there are several complete chains running side 
by side. All which facts imply a metamorphosis of the 
elongated cells, caused by the continued action of the currents 
carried. 


§ 281. Here, however, presents itself a further problem. 
Taking it as manifest that there is a typical distribution of 
supporting tissue adapted to meet the mechanical strains a 
plant is exposed to by its typical mode of growth, and also 
that there goes on special adaptation of the supporting tissue 
to the special strains the individual plant has to bear; and 
taking it as tolerably evident that the sap channels are 
originally determined by the passage of currents along lines 
of least resistance; there still remains the ultimate question— 
Through what physical actions are established these general 
and special adjustments of supporting tissue to the strains 
borne, and these distributions of nutritive liquid required to 
make possible such adjustments? Clearly, if the external 
actions produce internal reactions; and if this play of actions 
and reactions results in a balancing of the strains by the 
resistances; we may rationally suspect that the incident 
forces are directly conducive to the structural changes by 
which they are met. Let us consider how they must work. 


THE INNER TISSUES OF PLANTS. 269 


When any part of a plant is bent by the wind, the tissues 
on its convex surface are subject to longitudinal tension, and 
these extended outer layers compress the layers beneath 
them. Such of the vessels or canals in these subjacent layers 
as contain sap, must have some of this sap expelled. Part of 
it will be squeezed through the more or less porous walls of 
the canals into the surrounding tissue, thus supplying it 
with assimilable materials; while part of it, and probably 
the larger part, will be thrust along the canals longitudinally 
upwards and downwards. When the branch or twig or leaf- 
stalk recoils, these vessels, relieved from pressure, expand to 
their original diameters. As they expand, the sap rushes 
back into them from above and below. In whichever of 
these directions least has been expelled by the compression, 
from that direction most must return during the dilation ; 
seeing that the force which more efficiently resisted the 
thrusting back of the sap is the same force which urges it 
into the expanded vessels again, when they are relieved from 
pressure. At the next bend of the part a further portion of 
‘sap will be squeezed out, and a further portion thrust for- 
~vards along the vessels. This rude pumping process thus 
‘serves for propelling the sap to heights which it could not 
reach by capillary action, at the same time that it incident- 
ally serves to feed the parts in which it takes place. It 
strengthens them, too, just in proportion to the stress to be 
borne; since the more severe and the more repeated the 
strains, the greater must be the exudation of sap from the 
vessels or ducts into the surrounding tissue, and the greater 
the thickening of this tissue by secondary deposits. By 
this same action the movement of the sap is determined 
either upwards or downwards, according to the conditions. 
While the leaves are active and evaporation is going on from 
them, these oscillations of the branches and petioles urge 
forward the sap into them; because so long as the vessels of 
the leaves are being emptied, the sap in the compressed 
vessels of the oscillating parts will meet with less resistance 


270 PHYSIOLOGICAL DEVELOPMENT. 


in the direction of the leaves than in the opposite direction. 
But when evaporation ceases at night, this will no longer be 
the case. The sap drawn to the oscillating parts, to supply 
the place of the exuded sap, must come from the directions 
of least resistance. A slight breeze will bring it back from 
the leaves into the gently-swaying twigs, a stronger breeze 
into the bending branches, a gale into the strained stem and 
roots—roots in which longitudinal tension produces, in 
another way, the same effects that transverse tension does in 
the branches. ; 

Two possible misinterpretations must be guarded against. 
It must not be supposed that this force-pump action causes 
movement of the sap towards one point rather than 
another: it is simply an aid to its movement. From the 
stock of sap distributed through the plant, more or less is 
everywhere being abstracted—here by evaporation; here by 
the unfolding of the parts into their typical shapes; here by 
both. The result is a tension on the contained liquid columns, 
that is greatest now in this direction and now in that. This 
tension it is which must be regarded as the force that 
determines the current upwards or downwards; and all which 
the mechanical actions do is to facilitate the transfer to the 
places of greatest demand. Hence it happens that in a plant 
prevented from oscillating, but having a typical tendency to 
ussime a certain height and bulk, the demands set up by its 
unfolding parts will still cause currents; and there will still 
be. alternate ascents and descents, according as the varying 
conditions. change the direction of greatest demand—the 
only difference being, that in the absence of oscillations the 
the growth will be less vigorous. Similarly, it must 
not be supposed that mechanical actions are here alleged to be 
the sole causes of wood-formation in the individual plant. The 
tendency of the individual plant to form wood at places where 
wood has been habitually formed by ancestral plants, is. 
manifestly a cause, and, indeed, the chief cause. In this, as. 
in all other cases, inherited structures repeat themselves 


THE INNER TISSUES OF PLANTS. 271 


irrespective of the circumstances of the individual: absence 
of the appropriate conditions resulting simply in imperfect 
repetition of the structures. Hence the fact that in trained 
trees and hothouse shrubs, dense substance is still largely 
deposited; though not so largely as where the normal me- 
chanical strains have acted. Hence, too, the fact, that 
in such plants as the Elephants-foot or the  Welwitschia 
mirabilis, which for untold generations can have undergone 
no oscillations, there is an extensive formation of wood 
(though not to any considerable height above the ground), in 
repetition of an ancestral type: natural selection having 
here maintained the habit as securing some other advantage 
than that of support. | 

Still, it must be borne in mind that though intermittent 
mechanical strains cannot be assigned as the direct causes of 
these internal differentiations in plants that are artificially 
sheltered or supported, they are assignable as the indirect 
causes; since the inherited structures, repeated apart from 
such strains, are themselves interpretable as accumulated 
results of such strains acting on successive generations of 
ancestral plants. This will become clear on combining the 
several threads of the argument and bringing it to a close, 
which we may now do. 


§ 282. To put the co-operative actions in their actual order, 
would-require us to consider them as working on individuals 
small modifications that become conspicuous and definite 
only by inheritance and gradual increase ; but it will aid our 
comprehension without leading us into error, if we suppose the 
whole process resumed in a single continuously-existing plant. 

As the plant erects the integrated series of fronds whose 
united parts form its rudimentary axis, the increasing area 
of frond-surface exposed to the sun’s rays entails an increasing 
draught upon the liquids contained in the rudimentary 
axis. The currents of sap so produced, once established along 
certain lines of cells that offer least resistance, render them 


272 FHYSIOLOGICAL DEVELOPMENT. 


by their continuous passage more and more permeable. This 
establishment of channels is aided by the wind. Each bend 
produced by it while yet the tissue is undifferentiated, 
squeezes towards the place of growth and evaporation the 
liquids that are passing by osmose from cell to cell; and 
when the lines of movement become defined, each bend helps, 
by forcing the liquid along these lines, to remove obstructions 
and make continuous canals. As fast as this transfer of sap 
is facilitated, so fast is the plant enabled further to raise itself, 
and add to its assimilating surfaces; and so fast do the 
transverse strains, becoming greater, give more efficient 
aid. The channels thus formed can be neither in the 
centre of the rudimentary axis nor at its surface; for at 
neither of these places can the transverse strains produce 
any considerable compressions. They must arise along a tract 
between the outside of the axis and its core—a tract along 
which there occur the severest squeezes between the ex- 
tended outer layers and the internal mass. Just that dis- 
tribution which we find, is the distribution which these me- 
chanical actions tend to establish. 

As the plant gains in height, and as the mass of its foliage 
accumulates, the strains thrown upon its axis, and especially 
the lower part of its axis, rapidly increase. Supposing the 
forms to remain similar, the strains must increase in the ratio 
of the cubes of the dimensions ; or even in a somewhat higher 
ratio. One consequence must be, that the compressions to 
which the vessels at the lower part of the stem are subject, 
become greater as fast as the height to which the sap has to 
be raised becomes greater; and another consequence must be, 
that the local exudation of sap produced by the pressure is 
propcrtionately augmented. Hence the materials for nutri- 
tion of the surrounding tissues being there supplied more 
abundantly, we may expect thickening of the surrounding 
tissues to show itself there first: in other words, wood 
will be formed round the vessels of the lower part of 
the stem. The resulting greatcr ability of this lower 


THE INNER TISSUES OF PLANTS. Pats: 


part of the stem to bear strains, renders possiblé an increase 
of height; and while after an increase of height the lowest 
part becomes still further strained, and still further thickens, 
the part above it, exposed to like actions, undergoes a like 
thickening. This induration, while it spreads upwards, 
also spreads outwards. As fast as the rude cylinder of dense 
matter formed in this way, begins to inclose the original 
vessels, it begins to play the part of a resistant mass, between 
which and the outer layers the greatest compression occurs 
at each bend. While, therefore, the original vessels become 
useless, the peripheral cells of the developing wood become 
those which have their liquid contents squeezed out longitu- 
dinally and laterally with the greatest force; and, consequently, 
amid them are formed new sap-channels, from which there is 
the most active local exudation, producing the greatest 
deposit of dense matter. 

Thus fusing together, as it were, the individualities of 
successive generations of plants, and letting that facilitation 
of the process which natural selection has all along given, 
be represented by the most favourable working together of 
these mechanical processes, we are enabled to interpret 
the leading internal differentiations of plants as consequent 
on a direct equilibration between inner~and outer forces. 
Here, indeed, we see illustrated in a way more than usually 
easy to follow, the eventual balancing of outer actions by 
inner reactions. The relation between the demand for liquid 
and the formation of channels that supply liquid, as well 
as that between the incidence of strains and the deposit 
of substance that resists strains, are among the clearest special 
examples of the general truth that the moving equilibrium 
of an organism, if not overthrown by an incident force, must 
eventually be adjusted to it. 

The processes here traced out are, of course, not to be 
taken as the only differentiating processes to which the 
inner tissues of plants have been subject. Besides the chief 


changes we have considered. various less conspicuous changes 
18 
VoL. I 


274 PHYSIOLOGICAL DEVELOPMENT. 


have taken place. These must be passed over as arising 
in ways too involved to admit of specific interpreta- 
tions; even supposing them to have been produced by 
causes of the kind assigned. But the probability, or 
rather indeed the certainty, 1s, that some of them have not 
been so produced.. Here, as in nearly all other cases, in- 
diect requilibration has worked in aid of direct equilibration ; 
and in many cases indirect equilibration has been the sole 
agency. Jesides ascribing to natural selection the rise of 
various internal modifications of other classes than those 
above treated, we must ascribe some even of these to natural 
selection. It is so with the dense deposits which form 
thorns and the shells of nuts: these cannot have resulted 
from any inner reactions immediately called forth by outer 
actions ; but must have resulted mediately through the effects 
of such outer actions on the species. Let it be understood, 
therefore, that the differentiations to which the foregoing 
interpretation applies, are only those most conspicuous ones 
which are directly related to the most conspicuous in- 
cident forces. They must be taken as instances on the 
strength of which we may conclude that other internal 
differentiations have had a natural genesis, though in ways 
that we cannot trace. 


CHAPTER V. 
PHYSIOLOGICAL INTEGRATION IN PLAN'S. 


§ 283. A good deal has been implied on this topic in the 
preceding chapters. Here, however, we must for a brief 
space turn our attention immediately to it. 

Plants do not display integration in such distinct and 
‘multiplied ways as do animals. But its advance may be 
traced both directly and indirectly—directly in the increas- 
ing co-ordination of actions, and indirectly in the effect of 
this upon the powers and habits. 

Let us group the facts under these heads: ascending in 
both cases from the lower to the higher types. 


§ 284. The inferior Alga, along with little unlikeness of 
parts, show us little mutual dependence of parts. Having 
surfaces similarly circumstanced everywhere, much physio- 
logical division of labour cannot arise; and therefore there 
cannot be much physiological unity. Among the superior 
Algae, however, the differentiation between the attached part 
and the free part is accompanied by some integration. There 
is evidently a certain transfer of materials, which is doubtless 
facilitated by the elongated forms of the cells in the stem, 
and probably leads to the formation of dense tissue at the 
places of greatest strain, in a way akin to that recently ex- 
plained in other cases. And where there is this co-ordina- 
tion of actions, the parts are so far mutually dependent that 


each dies if detached from the other. That though the 
18 


276 PHYSIOLOGICAL DEVELOPMENT. 


organization is so low neither part can reproduce the other 
and survive by so doing, is probably due to the circumstance 
that neither part contains any considerable stock of untrans- 
formed protoplasm, out of which new tissues may be pro- 
duced. 

Fungi and Lichens present no very significant advances 
of integration. We will therefore pass at once to the 
Acrogens. In those of them which, either as single fronds 
or strings of fronds, spread over surfaces, and which, rooting 
themselves as they spread, do not need that each part should 
receive aid from remote parts, there is no developed vascular 
system serving to facilitate transfer of nutriment: the parts 
being little differentiated there is but little integration. ut 
along with assumption of the upright attitude and the ac- 
companying specializations, producing vessels for distribu- 
ting sap and hard tissue for giving mechanical support, there 
arises a decided physiological division of labour; rendering 
the aérial part dependent on the imbedded part and the im- 
bedded part dependent on the aérial part. Here, indeed, as 
elsewhere, these concomitant changes are but two aspects of 
the same change. Always the gain of power to discharge a 
special function involves a loss of power to perform other 
functions ; and always, therefore, increased mutual dependence 
constituting physiological integration, must keep pace with 
that increased fitting of particular parts to particular duties 
which constitutes physiological differentiation. 

Making a great advance among the Acrogens, this physio- 
logical integration reaches its climax among Endogens and 
Exogens. In them we see interdependence throughout 
masses that are immense. Along with specialized appli- 
ances for support and transfer, we find an exchange of aid at 
great distances. We see roots giving the vast aérial growth 
a hold tenacious enough to withstand violent winds, and 
supplying water enough even during periods of drought; we 
see a stem and branches of corresponding strength for up- 
holding the assimilating organs under ordinary and extraor- 


PHYSIOLOGICAL INTEGRATION IN PLANTS. 277 


dinary strains; and in these assimilating organs we sce 
elaborate appliances for yielding to the stem and roots the 
materials enabling them to fulfil their offices. As a con- 
sequence of which greater integration accompanying the 
greater differentiation, there is ability to maintain life over 
an immense period under marked vicissitudes. 

Even more conspicuously exemplified in Pheenogams, is that 
physiological integration which holds together the functions 
not of the individual only but of the species as a whole. The 
organs of reproduction, both in their relations to other parts 
of the individual bearing them and in their relations to 
corresponding parts of other individuals, show us a kind of 
integration conducing to the better preservation of the race ; 
as those already specified conduce to the better preservation of 
the individual. In the first place, this greater co-ordination 
of functions just described, itself enables Pheenogams to be- 
queath to the germs they cast off, stores of nutriment, pro- 
tective envelopes, and more or less of organization: so giving 
them greater chances of rooting themselves. In the second 
place, certain differentiations among the parts of fructification, 
the meaning of which Mr. Darwin has so admirably explained, 
give to the individuals of the species a kind of integration 
that makes possible a mutual aid in the production of 
vigorous offspring. And it is interesting to observe how, in 
that dimorphism by which in some eases this mutual aid is 
made more efficient, the greater degree of integration is 
dependent on the greater degree of differentiation—not simply 
differentiation of the fructifying organs from other parts of the 
plant bearing them, but differentiation of these fructifying 
organs from the homologous organs of neighbouring indi- 
viduals of the same race. Another form of this 
co-ordination of functions that conduces to the maintenance of 
the species, may be here named—partly for its intrinsic | 
interest. I refer to the strange processes of multiplication 
that occur in the genus Bryophyllum. It is well known that 
the succulent leaves of B. calycinum, borne on foot-stalks 


278 PHYSIOLOGICAL DEVELOPMENT, 


so brittle that they are easily snapped by the wind, send 
forth from their edges when they fall to the ground, buds 
that root themselves and grow into independent plants. The 
correlation here obviously furthering the preservation of the 
race, is more definitely established in another species of the 
genus—B. proliferum. This plant, shooting up to a consider- 
able height, and having a stem containing but little woody 
fibre, habitually breaks near the bottom while still in flower ; 
and is thus generally prevented from ripening its seeds. The 
multiplication is, however, secured in another way. Before 
the stem is broken young plants have budded out from the 
pedicels of the flowers, and have grown to considerable lengths ; 
and on the fall of the parent they forthwith commence their 
separate lives. Here natural selection has established a 
remarkable kind of co-ordination between a special habit of 
growth and decay, and a special habit of proliferation. 


§ 285. The advance of physiological integration among 
plants as we ascend to the higher types, is implied by their 
greater constancy of structure, as well as by the stricter limi- 
tation of their habitats and modes of life. ‘‘ Complexity of 
structure is generally accompanied with a greater tendency 
to permanence in form,” says Dr. Hooker; or, conversely, 
“the least complex are also the most variable.” This is the 
second aspect under which we have to contemplate the facts. 

The differences between the simpler A/ge and Fungi, and 
between them and the Lichens, are so feebly marked that 
botanists have been unable to frame satisfactory definitions 
of these classes. ‘‘ Linneeus, for instance, and Jussieu, con- 
sidered Lichens as forming a part of A/ye, in which they 
are followed by Fries.” Mr, Berkeley, however, quoting the 
admission of Fries “that there is no certain distinction be- 
tween Lichens and Fungi, except the presence in the former 
of green globules, resembling grains of chlorophyll,” him- 
self prefers to unite Hungi and Lichens under the general 
head of Mycetales. This structural indefiniteness is accom- 


PHYSIOLOGICAL INTEGRATION IN PLANTS. 279 


panied by functional indefiniteness. Though, considered 
collectively, these Thallogens form ‘three very natural 
groups, according as they inhabit the water, the earth, or 
the air ;” yet if, instead of their higher members we look at 
their lower members, we find these distinctions of habitat very 
undecided. -A/ge, which are mostly aquatic, include many 
small forms that frequent the damp places preferred by 
Lichens and Fungi. Among Lichens, as among Fungi, there 
are kinds that lead submerged lives like the Alge. While 
terrestrial Lichens and Fungt compete for the same places, as 
well as simulate one another’s modes of growth. Besides 
this indistinctness of the classes, there is great variability in 
the shapes and modes of life of their species—a variability 
so great that what were at first taken to be different species, 
or different genera, or even different orders, have proved to 
be merely varieties of one species. So inconstant in struc- 
ture are the A/ge that Schleiden quotes with approval the 
opinion of Kutzing, that “there are no species but merely 
forms of Alge.”’ In all which groups of facts we see that 
these lowest types of plants, little differentiated, are also but 
little integrated. 

Acrogens present a parallel relation between the small 
specialization of functions which constitutes physiological 
differentiation, and the small combination of functions which 
constitutes physiological integration. ‘ Mosses,” says Mr. 
Berkeley, ‘“‘are no less variable than other cryptogams, 
and are therefore frequently very difficult to distinguish. 
Not only will the same species exhibit great diversity 
in the size, mode of branching, form and nervation of the 
leaves, but the characters of even the peristome itself are 
not constant.” And concerning the classification of the 
remaining group, Jticales, he says:—‘ Not only is there 
great difficulty in arranging ferns satisfactorily, but it is 
even more difficult to determine the limits of species.”’ 

After this vagueness of separation as well as inconstancy 
of structure and habit among the lower plants, the stability 


280 PHYSIOLOGICAL DEVELOPMENT. 


of structure and habit and divisibility of groups among 
the higher plants, appear relatively marked. Though 
Pheenogams are much more variable than most botanists have 
until recently allowed, yet the definitions of species and 
genera may be made with far greater precision and are 
far less capable of change than among Cryptogams. 
And this comparative fixity of type, implying, as it 
does, a closer combination of the component functions, we 
see to be the accompaniment of the greater differentiation of — 
those functions and of the structures performing them. That 
these characters are correlatives is further shown by the 
fact that the higher plants are more restricted in their 
habitats than the lower plants, both in space and time. “ The 
much narrower delimitation in area of animals than plants,” 
says Dr. Hooker, ‘‘and greater restriction of Faunas than 
Floras, should lead us to anticipate that plant types are, 
geologically speaking, more ancient and permanent than the 
higher animal types are, and so I believe them to be, and L 
would extend the doctrine even to plants of highly complex 
structure.” ‘Those classes and orders which are the least 
complex in organization are the most widely distributed.” 


§ 286. Thus that which the general doctrine of evolution 
leads us to anticipate, we find implied by the facts. The 
physiological division of labour among parts, can go on only 
in proportion to the mutual dependence of parts; and the 
mutual dependence of parts can progress only as fast as there 
arise structures by which the parts are efficiently combined, 
and the mutual utilization of their actions made easy. 

To say definitely by what process is brought about this 
co-ordination of functions which accompanies their specializa- 
tion, is hardly practicable. Direct and indirect equilibration 
doubtless co-operate in establishing it. We may see, for 
example, that every increase of fitness for function produced 
in the aérial part of a plant by light, as well as every increase 
of fitness for function produced in its imbedded part by the 


PHYSIOLOGICAL INTEGRATION IN PLANTS, 281 


direct action of the moist earth, must conduce to an increased 
current of the liquid evaporated from the one and supplied 
by the other—must serve, therefore, to aid the formation of 
sap-channels in the ways already described; that is—must 
serve to develop the structures through which mutual aid of 
the parts is given: the additional differentiation tends imme- 
diately to bring about the additional integration. Con- 
trariwise, it 1s obvious that an interdependence such as we 
see between the secretion of honey and the fertilization of 
germs, or between the deposit of albumen in the cotyledons 
of an embryo-plant and the subsequent striking root, is a kind 
of integration in the actions of the individual or of the 
species, which no differentiation has a direct tendency to 
initiate. Hence we must regard the total results as due to a 
plexus of influences acting simultaneously on the individual 
and on the species: some chiefly affecting the one and some 
chiefly affecting the other. 


CHAPTER VI. 


DIFFERENTIATIONS BETWEEN THE OUTER AND INNER 
TISSUES OF ANIMALS. 


§ 287. What was said respecting the primary physiological 
differentiation in plants, applies with little beyond change of 
terms to animals. Among Protozoa, as among Protophyta, 
the first definite contrast of parts that arises is that between 
outside and inside. The speck of jelly or sarcode which appears 
to constitute the simplest animal, proves, on closer examina- 
tion, to be a mass of substance containing a nucleus—a 
periplast in the midst of which there is a minute endoplast, 
consisting of a spherical membrane and its contents. 

This parallel, only just traceable among these Rhizopods, 
which are perpetually changing the distribution of their outer 
substance, becomes at once marked in those higher Protozoa 
which have fixed shapes, and maintain constant relations 
between their surfaces and their environments. Indeed the 
Rhizopods themselves, on passing into a state of quiescence 
in which the relations of outer and inner parts are fixed, 
become encysted: there is formed a hardened outer coat 
different from the matter which it contains. And what is 
here a temporary character answering to a temporary 
definiteness of conditions, is in the Jnfusoria a constant 
character, answering definite conditions that are constant. 
Each of these minute creatures, though not coated by a dis- 
tinct membrane, has the outer layer of its sarcode indurated : 
the indurated substance being not separable from the sub- 
stance inclosed, but passing into it insensibly. 


THE OUTER AND INNER TISSUES OF ANIMALS. 282 
-- § 288. The early establishment of this primary contrast of 
tissues answering to this primary contrast of conditions, is no 
less conspicuous in aggregates of the second order. The 
feebly-integrated units of a Sponge, with individualities so 
little merged in that of the whole they form that most of 
them still retain their separate activities, nevertheless show 
us, in the unlikeness that arises between the outermost layer 
and the contained mass, the effect of converse with unlike 
conditions. This outermost layer is composed of units some- 
what flattened and united into a continuous membrane—a 
kind of rudimentary skin. 

Secondary aggregates in which the lives of the units are 
more subordinate to the life of the whole, carry this dis- 
tinction further. The leading physiological trait of every 
ceelenterate animal is the divisibility of its substance into 
endoderm and ectoderm—the part next the food and the part 
next the environment. Fig. 147, rudely representing a por- 
tion of the body-wall of a Hydra seen in section, gives some 
idea of this fundamental differentiation. ‘The creature con- 
sists of a simple sac, the cavity of which is in direct commu- 
nication with the surrounding water; and hence there is but 
little unlikeness between the outer and inner layers: indeed 
they are said to be capable of exchanging their functions. 
The essential contrast is that between the parts in contact 
with foreign substances and the parts sheltered from them— 
between the developed surfaces of the endoderm and ectoderm, 
and that intermediate stratum of nucleated sarcode from 
which the two grow in opposite directions. 

Between this case and the case of the Sponge, we may 
readily trace the connexion. Suppose a mass of Amaba-form 
units, the outermost of which are united into a layer analogous 
to that by which a living Sponge is covered, to be represented 
‘by a lump of plastic clay ; and for convenience of identifica- 
tion, suppose the surface of the clay to be coated by an 
extensible film, say of caoutchouc. Let this clay, so coated, 
be moulded into the shape of a cup; the cup be gradually 


284 PHYSIOLOGICAL DEVELOPMENT. 


deepened until it becomes jar-shaped ; and finally, by narrow- 
ing its neck, vase-shaped. And conceive the stretched film 
to continue everywhere covering the surface during these 
changes of form. What will finally be the relations of the 
parts to one another? The caoutchouc will line the inside of 
the vase as well as coat its outside. The vase will consist of 
a stratum of the clay included between the two India-rubber 
surfaces. We shall have a distribution of layers answering 
completely to the distribution of tissues in the Hydra. Now 
if we imagine that this artificial layer which has covered the 
clay during its changes of form, is produced by transforma- 
tion of the clay, we shall see that when the mass is changed 
into the vase-shape, the surfaces that have become outer and 
inner will develop in opposite directions from the substance 
lying between them; just as do the Hydra’s ectoderm and 
endoderm. And if, once more, we conceive these outer and 
inner surfaces so resulting, to be affected by conditions some- 
what unlike—the one by matters placed in the jar, and the 
other by the medium surrounding the jar—we shall have, in 
the slight difference produced between them, a difference 
corresponding to that between the surfaces of the Hydra’s 
stomach and skin. 

Besides being able thus to understand how an aggregate 
of Ameba-form units, originally coated by a single layer, 
may pass into an aggregate composed of a double layer; we 
may also understand under what influences the transition 
takes place. If the habit which some of the primary aggre- 
gates have, of wrapping themselves round masses of nutri- 
ment, is followed by a secondary aggregate, there will 
naturally arise just that re-differentiation which the Hydra 
shows us. 


§ 289. These duplicated surfaces which we see in every simple 
ceelenterate animal, are re-duplicated in all animals of higher 
classes—the more developed Celenterata themselves showing 
us the transition. ‘Compared with the Hydroid Polypes,” 


THE OUTER AND INNER TISSUES OF ANIMALS. 285 


says Prof. Huxley, “the higher forms are double animals, 
and a section of their bodies is, morphologically speaking, like 
a section of two Hydre, one contained within the other.” 
The relations of the parts may be illustrated thus :—Cut off 
the finger of a leather glove that has a lining; and let the 
leather and the lining represent the ectoderm and endoderm 
of a Hydra. Thrust the point of the glove-finger back into 
the cavity, until the introverted portion comes out bevond 
the open end. Cut off the projecting apex of the introverted 
portion level with the edges of the open end; and then unite 
the edges of the introverted portion and the outer portion. 
The arrangement of structures will then typify that which is 
common to all animals except the Protozoa and the lower 
Coelenterata : the introverted part representing the alimentary 
canal; the outer part representing the body-wall; and the 
closed cavity between the two representing the peri-visceral 
sac. This, however, is not the whole parallelism. If in the 
glove-finger, representing in its original form the Hydra, we 
suppose the leather standing for the ectoderm to be growing 
outwards, and the lining standing for the endoderm to be 
growing inwards, then if in the part that is introverted the 
same relations of growth are maintained, it is manifest that 
of its two layers the one which was outermost and is now 
innermost, will grow towards the open cavity which stands | 
for the alimentary canal, while the other layer will grow 
towards the closed cavity standing for the peri-visceral sac. 
And these are the directions of growth actually found in the 
parts thus symbolized. 

This simile must not have more meaning given to it than 
is intended. Though there is reason for suspecting that a 
re-duplication has taken place in the course of evolution, and 
that the peri-visceral sac which distinguishes all the higher 
classes of animals from the lower, has been formed by it; yet 
the method of re-duplication cannot have been anything like 
that described ; and has probabiy been so different a one as 
to negative the implied homologies of the layers. The illus- 


286 PHYSIOLOGICAL DEVELOPMENT. 


tration is here used merely to convey, in a way easy to’ 
follow, an idea of the relations between outer and inner 
tissues, as they exist in the more complex animals. The two 
facts which we have to note are these :—First that, as Prof. 
Huxley points out in his essay on “ Tegumentary Organs,” 
the course of differentiation in the body-wall of the Hydra, is 
paralleled by the course of differentiation in the skin of every 
more complex animal up to the highest mammal. Between 
the epidermis and the derma there is a layer of indifferent 
tissue corresponding to the layer that lies between the endo- 
derm and ectoderm of the Hydra; and from this layer, as 
from its homologue, the differentiations proceed in opposite 
directions. Though the resulting two layers, exposed to 
more unlike conditions than those of the Hydra, are more 
unlike one another, yet we see in them essentially the same 
course of metamorphosis and the same subordination, of it to 
the relations of outside and inside. In the second place, we 
have to note that the wall of the alimentary canal, though it 
ig in one sense internal by contrast with the skin as external, 
and is correspondingly differentiated from the skin, is in 
another sense like the skin, in having one surface in contact 
with foreign substances (presented as food) and the other 
surface in contact with the living substance of the body; and 
that consequently it undergoes, like the skin, a differentia- 
tion into two layers, one growing towards the relatively 
external or food-containing cavity, and the other towards the 
rigorously internal cavity—the closed peri-visceral sac. 


§ 290. Whether direct equilibration or indirect equilibra- 
tion has had the greater share in producing this universally- 
present contrast between the inner and outer tissues of 
animals, must be left undecided. The two causes have all 
along co-operated—modification of the individual accumu- 
lated by inheritance predominating in some cases, and in 
other cases modification of the race by survival of the inci- 
dentally fittest. On the one hand, the action of the medium 


THE OUTEK AND INNER TISSUES OF ANIMALS. 287 


on the organism cannot fail to change its surface more 
than its centre, and so differentiate the two; while on the 
other hand, the surfaces of organisms inhabiting the same 
medium display extreme unlikenesses which cannot be due to 
the immediate actions of their medium. Let us dwell a 
moment on the antithesis. 

We have abundant evidence that animal protoplasm is 
rapidly modified by light, heat, air, water, and the salts 
contained in water—coagulated, turned from soluble into in- 
soluble, partially changed into isomeric compounds, or other- 
wise chemically altered. Immediate metamorphoses of this 
kind are often obviously produced in ova by changes of their 
media. At the outset, therefore, before yet there existed 
any such differentiation as that which now usually arises by 
inheritance, these environing agencies must have tended to 
originate a protective envelope. lor a modification produced 
by them on the superficial part of the protoplasm, must 
either have been a decomposition or else the formation of a 
compound that remained stable under their subsequent action. 
There would be generated an outer layer of substance that 
was so molecularly immobile as to be incapable of further 
metamorphoses, while it would shield the contained proto- 
plasm from that too great action of external forces which, by 
rapidly changing the unstable equilibrium of its molecules 
into a relatively stable equilibrium, would arrest development. 
Evidently organic evolution, whether individual or general, 
must always and everywhere have been subordinate to these 
physical necessities. Though natural selection, beginning 
with minute portions of protoplasm, must all along have 
tended to establish a molecular composition apt to undergo 
this differentiation of surface from centre to the most favour- 
able extent, yet it must all along have done so while con- 
trolled by this process of direct equilibration. 

Contrariwise, the many and great, unlikenesses among the 
dermal structures of creatures inhabiting the same element, 
cannot be ascribed to any such cause. The contrasts between 


288 PHYSIOLOGICAL DEVELOPMENT. 


naked and shelled Gastropods, between marine Worms and. 
Crustaceans, between soft-skinned Fish and Fishin armour like 
the Péericthys, must have been produced entirely by natural 
selection. Environing forces are, as before, the ultimate 
causes; but the forces are now not so much those exercised 
by the medium as those exercised by the other inhabitants of 
the medium ; and they do not act by modifying the surface 
of the individual, but by killing off individuals whose surfaces 
are least fitted to the requirements: thus slowly affecting the 
species. The dermal skeleton bristling with spines, which 
protects the Diodon or the Cyclicthys from enemies it could 
not escape, still comes within the general formula of an outer 
tissue differentiated from inner tissues by the outer actions to 
which the creature is exposed—the differentiation having 
gone on until there is equilibrium between the destructive 
forces to be met and the protective forces which meet them. 

If we venture to apportion the respective shares which 
mediate and immediate actions have had in differentiating 
outer from inner tissues, we shall probably not be far wrong 
in ascribing that part of the process which is alike in all 
animals, mainly to the direct actions of their media; while 
we ascribe the multitudinous unlikenesses of the process in 
various animals, partly to the indirect actions of the media, 
and partly to the indirect actions of other animals by which 
the media are inhabited. That is to say, while assigning the 
specialities of the differentiations to the specialities of con- 
verse with the agencies in the environment, most of them 
organic, we may assign to the constant and universal con- 
verse with its inorganic agencies, that universal characteristic 
of tegumentary structures—their development into a double 
layer separated by undifferentiated substance, from which the 
outermost grows outwardly and the innermost grows in- 
wardly. 

Here let me add a piece of evidence which strengthens 
very greatly the general argument, at the same time that it 
justifies this apportionment. When ulceration has gone deep 


THE OUTER AND INNER TISSUES OF ANIMALS, 289 


enough to destroy the tegumentary structures, these are never 
reproduced. The puckered surface formed where an ulcer 
heals, consists of modified connective tissue, which, as the 
healing goes on, spreads inwards from the edges of the ulcer 
—some of it, perhaps, growing from the portions of connective 
tissue that dip down between the muscular bundles. This 
connective tissue, mark, out of which is thus constituted the 
make-shift skin, is normally covered by both the epidermis 
and that stratum of indifferent tissue from which the growth 
proceeds in opposite directions—is the inner layer that grows 
inwardly. What has happened to it? It has now become 
the outermost layer. And how does it comport itself under 
its new conditions? It produces a layer that plays the part 
of epidermis and grows outwardly. For since the surtace, 
subject to friction and exfoliation, has to be continually 
renewed, there must be a continual reproduction of a super- 
ficial layer from a layer beneath. That is to say, the con- 
tact of this deep-seated tissue with outer agencies, produces 
in it some approach towards that composition which we find 
universally characterizes outer-tissue—a protomorphic layer, 
which differentiates in opposite directions. But while we see 
under this exposure to the conditions common to all integu- 
ment, a tendency to assume the structure common to all 
integument, we see no tendency to assume any of the 
specialities of tegumentary structure: no rudiments of glands 
or hair sacs make their appearance. 

This apportionment we shall see the more reason to accept 
as approximately expressing the truth, on remembering that 
the mode of differentiation of outer from inner tissues which 
is common to all animals is common to all plants; and 
on observing, further, that the more special interpretation 
suggested as not improbable in the case of plants, is not 
improbable in the case of animals. For as it was argued 
that in plants the forces evolved from within the organism, 
and the forces falling on it from without, must have some 


place between centre and surface at which they balance; and 
VOL. IL. 19 


290 PHYSIOLOGICAL DEVELOPMENT. 


that at this place will lie the unstable protoplasm that 
develops outwardly into a substance which is stable in face 
of outer forces, and inwardly into a substance which is stable 
in face of inner forces; so in animals, we may regard this 
universally-present layer whence epidermis grows outwardly 
and connective tissue inwardly, as similarly the place of 
equilibrium between these antagonist forces. And for this 
d priort interpretation we may indeed, among animals, find 
a posteriort warrant. We have but to increase the mechanical 
action or chemical irritation at some part of an animal’s 
surface, to make this plane of indifferent tissue retreat in- 
wardly ; for to say that the epidermis becomes thicker, is, in 
mechanical terms, to say that the place of equilibrium between 
outer and inner forces is further from the surface, 


CHAPTER VII. 
DIFFERENTIATIONS AMONG THE OUTER TISSUES OF ANIMALS, 


§ 291. The outer tissues of animals, originally homo- 
geneous over their whole surfaces, pass into a heterogeneity 
which fits their respective parts to their respective conditions. 
So numerous and varied are the implied differentiations, that 
it is impracticable here to deal with them all even in outline. 
To trace them up through classes of animals of increasing 
degrees of aggregation, would carry us into undue detail. 

Did space permit, it would be possible to point out among 
the Protozoa, various cases analogous to that of the Arcedia; 
which may be described as like a microscopic Limpet, having 
a sarcode body of which the upper surface has become horny, 
while the lower surface with its protruding pseudopodia, 
retains the primitive jelly-like character. That differentia- 
tions of this kind have been gradually established among 
these minute creatures through the unlike relations of their 
parts to the environment, is an inference supported by cases 
like that of Pamphagus—an intermediate form which is like 
the Amebda in having no carapace, but “ agrees with Arcella 
and Diffugia in having the pseudopodia protrusible from one 
extremity only of the body.” 

Many parallel specializations of surface among aggregates 
of the second order might be instanced from the Calenterata. 
In the Hydra, the ectoderm presents over its whole area no 
conspicuous unlikenesses; but there usually exist in the 


hydroid polypes of superior types, decided contrasts between 
19 * 


292 PHYSIOLOGICAL DEVELOPMENT. 


the higher and lower parts. While the higher parts retain 
their original characters, the lower parts excrete hard outer 
layers yielding support and protection. Various stages of 
the differentiation might be followed. “In Hydractinia,” 
says Prof. Green, this horny layer “becomes elevated at 
intervals to form numerous rough processes or spines, while 
over the general surface of the ectoderm its presence is 
almost imperceptible.” In other types, as in Cordylophora, 
it spreads part way up the animal’s sides, ending indefinitely. 
In Bimeria it “ extends itself so as to enclose the entire body 
of each polypite, leaving bare only the mouth and tips of the 
tentacles.”’ While in Campanularia it has become a partially- 
detached outer cell, into which the creature can retract its 
exposed parts. 

But it is as needless as it would be wearisome to trace 
through the several sub-kingdoms the rise of these multiform 
contrasts, with the view of seeking interpretations of them. 
It will suffice if we take a few groups of the illustrations 
furnished by the higher animals. 


§ 292. We may begin with those modifications of surface 
which subserve respiration. Though we ordinarily think of 
respiration as the quite special function of a quite special 
organ, yet originally it is not so. Little-developed animals 
part with their carbonic acid and absorb oxygen, through the 
general surface of the body. ven in the lower types of the 
higher classes, the general surface of the body aids largely in 
aérating the blood; and the parts that discharge the greater 
part of this function are substantially nothing more than 
slightly altered and extended portions of the skin. 

Such differentiations, marked in various degrees, are to be 
seen among Mollusca. In the Pteropoda the only modification 
which appears to facilitate respiration, is the minute vascularity 
of one part of the skin. In other types the specialized parts 
facilitating the exchange of gases, are those simple but 
numerous expansions of surface constituting the papille ; 


THE OUTER TISSUES OF ANIMALS. 293 


which, in the Ho/is and kinds allied to it, are distributed in 
rows or clusters all along the back. Instead of these, the 
Doris has appendages developed into elaborately-branched 
forms—small trees of blood-vessels covered by slightly- 
changed dermal tissues. And these arborescent branchiee are 
gathered together into a single cluster. Thus there is 
evidence that large external respiratory organs have arisen 
by degrees from simple skin: as, indeed, they do arise during 
the development of each individual having them. Just as 
gradually as in the embryo the simple bud on the integu- 
ment, with its contained vascular loop, passes by secondary 
buddings into a tree-like growth penetrated everywhere by 
dividing and sub-dividing blood-vessels; so gradually has 
there probably proceeded the differentiation which has turned 
part of the outer surface into an organ for excreting carbonic 
acid and absorbing oxygen. | 

Certain inferior vertebrate animals present us with a like 
metamorphosis of tissues. These are the Amphibia. The 
branchiz here developed from the skin are covered with cel- 
lular epidermis, not much thinner than that covering the rest 
of the body. Like it they have their surfaces speckled with 
pigment-cells; and are not even conspicuous by their extra 
vascularity—- where they are temporary at least. They facili- 
tate the exchange of gases in scarcely any other way than by 
affording a larger area of contact with the water, and inter- 
posing a rather thinner layer of tissue between the water 
and the blood-vessels. Those very simple branchie of the 
larval Amphibia that have them but for a short time, 
graduate into the more complex ones of those that have them 
for a long time or permanently ; showing, as before, the small 
stages by which this heterogeneity of surface accompanying 
heterogeneity of function may arise. 

In what way are such differentiations established? Partly, 
no doubt, by natural selection; but also to some degree, I 
think, by the inheritance of direct adaptations. That a por: 
tion of the integument at which aération is favoured by local 


294 PHYSIOLOGICAL DEVELOPMENT. 


conditions, should thereby be led to grow into a larger 
surface of aération, appears improbable: survival of those 
individuals which happen to have this portion of the integu- 
ment somewhat more developed, seems here the only likely 
cause. Nevertheless there is reason for suspecting that 
respiratory activity itself aids in the development of a re- 
spiratory appendage. The reason is this. Exchange of liquids 
through membrane depends on some difference, physical or 
chemical, between the liquids: if they are in all respects 
alike, and under equal pressures, no exchange will take place ; 
while, conversely, if they are much unlike there will be a 
rapid exchange. Now through the walls of capillaries, or 
through the sides of lacunze not yet developed into capillaries, 
there continually goes on an oozing both ways—from the 
blood into the tissues and from the tissues into the blood. 
By this double movement nutrition and depuration are alike 
made possible; and it is obvious both that in the absence of 
difference it would not occur, and that nothing would be 
gained if it did occur. Among other differences continually 
arising between the intra-vascular liquid and the extra- 
vascular liquid, is that due to their unlike charges of 
oxygen and carbonic acid. This difference, like other differ- 
ences, will cause exchange—the rapidity of the exchange 
doubtless being greater where the difference is greater. 
Hence if any part of an aquatic animal’s skin is nearest to 
the place where the blood has become most highly carbonized, 
or if it is so bathed with moving water that the plasma 
beneath its surface is more oxygenated than elsewhere, or 
both ; then, other things equal, this part of the skin will be 
the seat of an osmotic movement greater than goes on in the 
rest of the skin. But the exchange of oxygen for carbonic 
acid, proceeding faster here than elsewhere, will have for its 
accompaniment a more rapid exudation of nutritive matters. 
The liquid passing out of the blood-vessels to be replaced by 
the liquid passing into them, is a liquid containing the - 
substances that build up the surrounding tissues. Hence 


THE OUTER TISSUES OF ANIMALS. 295 


these tissues may be expected to grow: the area supplied by 
the increased currents of blood set up by this exchange, will 
become protuberant—will bud out; and the bud so formed 
will give origin to secondary buds at those parts of its surface 
which, as before, are most favourably circumstanced for 
carrying on the aération. Of course this process will be 
checked where, though otherwise advantageously placed, the 
growing branchiz would be specially liable to damage, or 
would be great hindrances to the creature’s movements. But 
bearing in mind that functionally-produced adaptation will 
here, as in other cases, be both aided and controlled by 
natural selection, we may ascribe to it an important share, if 
not a leading share, in the differentiation. 


§ 293. Among the conspicuous modifications by which the 
originally-uniform outer layer is rendered multiform, are the 
protective structures. Let us look first at the few cases in 
which the formation of these is ascribable mainly to direct 
equilibration. 

Already reference has been more than once made to those 
thickenings that occur where the skin is exposed to unusual 
pressure and friction. Are these adaptations inheritable ? 
and may they, by accumulation through many generations, 
produce permanent dermal structures fitted to permanent or 
frequently-recurring stress? Taking, for instance, the cal- 
losities on the knuckles of the Gorii/a, which are adapted to 
its habit of partially supporting itself on its closed hands 
when moving along the ground—shall we suppose that these 
defensive thickenings are produced afresh in each individual 
by the direct actions; or that they are inherited modifica- 
tions caused by such direct actions; or that they are wholly 
due to the natural selection of spontaneous variations? 
The last supposition does not seem a probable one; since it 
implies that those slight extra thicknesses of skin on the 
knuckles, with which we must suppose the selection to have 
commenced, were so advantageous as to cause survivals of the 


296 PHYSIOLOGICAL DEVELOPMENT. 


individuals having them. That survivals so caused, if they 
ever occurred at all, should have oecurred with the frequency 
requisite to establish and increase the variation, is hardly 
supposable. And if we reject, as also unlikely, the repro- 
duction of these callosities de movo in each individual, 
there remains only the inference that they have arisen 
by the transmission and accumulation of functional adapta- 
tions. Another case which seems interpretable 
only in an analogous way, is that of the spurs that are 
developed on the wings of certain birds—on those of the 
Chaja screamer for example. These are weapons of offence 
and defence. It is a familiar fact that many birds strike 
with their wings, often giving severe blows; and in the 
birds named, the blows are made more formidable by the 
horny, dagger-shaped growths standing out from those points 
on the wings which deliver them. Are these spurs directly 
or indirectly adaptive? To conclude that natural selection 
of spontaneous variations has caused them, is to conclude 
that, without any local stimulus, thickenings of the skin 
occurred symmetrically on the two wings at the places 
required ; that such thickenings, so localized, happened to 
arise in birds given to using their wings in fight; and that 
on their first appearance the thickenings were decided enough 
to give appreciable advantages to the individuals distinguished 
by them—advantages in bearing the reactions of the blows if 
not in inflicting the blows. But to conclude this is, I think, 
to conclude against probability. Contrariwise, if we assume 
that the thickening of the epidermis produced by habitual 
rough usage is inheritable, the development of these struc- 
tures presents no difficulty. The points of impact would 
become indurated in wings used for striking with unusual 
frequency. The callosities of surface thus generated, render- 
ing the parts less sensitive, would enable the bird in which 
they arose to give, without injury to itself, more violent blows 
and a greater number of them—so, in some cases, helping it 
fo conquer and survive. Among its descendants, inheriting 


THE OUTER TISSUES OF ANIMALS. 297 


the modification and the accompanying habit, the thickening 
would be further increased in the same way—survival of the 
fittest tending ever to accelerate the process. Presently the 
horny nodes so formed, hitherto defensive only in their 
effects, would, by their prominence, become offensive— would 
make the blows given more hurtful. And now natural 
selection, aiding more actively, would mould the nodes into 
spurs: the individuals in which the nodes were most pointed 
would be apt to survive and propagate; and the pointedness 
generation after generation thus increased, would end in the 
well-adapted shape we see. 

But if in these cases the differentiations which fit particular 
parts of the outer tissues to bear rough usage, are caused 
mainly by the direct balancing of external actions by in- 
ternal reactions, then we may suspect that the like is true 
of other modifications that occur where special strains and 
abrasions have to be met. Possibly it is true of sundry parts 
that are formed of hardened epidermis, such as the nails, 
claws, hoofs, and hollow horns of Mammals; “all of which,” 
says Prof. Huxley, “are constructed on essentially the same 
plan, being diverticula of the whole integument, the outer 
layer of whose ecderon has undergone horny metamorphosis.” 
Leaving open, however, the question what tegumentary 
structures are due to direct equilibration, furthered and con- 
trolled by indirect equilibration, it is tolerably clear that 
direct equilibration has been one of the factors. 

How has it produced its effects? that is to say—by what 
physical processes do pressure and friction bring about dermal 
hardenings? ‘To this inquiry there is an answer similar to 
that which was given to the inquiry respecting the formation 
of wood. (§ %80-2.) As in plants we saw that intermittent 
compressions of sap-canals increase the exudation of sap, and 
thus cause increased deposits of its contained substances in 
the surrounding tissues ; so in animals, we have good reason 
for concluding that intermittent compressions of the capil- 
laries increase the exudation of serum, and by thus supplying 


298 PHYSIOLOGICAL DEVELOPMENT. 


extra nutriment to the structures adjacent, lead, other things 
equal, to thickening or induration. The data for the con- 
clusion are these :—Through the walls of the capillaries the 
liquid plasma of the blood continually oozes. The oozing 
is partly osmotic and partly mechanical—partly due, that is, 
to the exchange of the unlike liquids that lie inside and out- 
side the capillaries, and partly to the greater pressure 
put upon the liquid inside. That this last is one of 
the causes is proved by the phenomena of dropsy—a disease 
in which the exudation is unduly rapid. Dropsy in the legs 
gets worse during the day, when by sitting and standing the 
weight of the blood to be borne by the vessels of the legs is 
increased; and gets better during the night, when by the 
recumbent attitude these vessels are relieved from this 
weight. Contrariwise, that cedematous swelling under the 
eyes which is common in the aged and debilitated, increases 
during the night and decreases during the day—gravitation 
serving, when the body is upright, to diminish the pressure of 
the blood at this part, and not having this effect when the 
body is horizontal. But if the plasma is to some extent 
forced through the walls of the capillaries by pressure, then 
not only will the action of the heart, aided at some parts by 
gravity, further the exudation, but the exudation will be 
furthered by external pressures from time to time falling on 
the capillaries. If the capillaries of the skin be squeezed 
by the thrust of some object against the surface, part of their 
contained blood will be driven back into the arteries, more 
will be driven forwards into the veins, and some will be made 
to exude. Immediately they are relieved from the pressure 
they will be refilled from the arteries, again to yield an extra 
portion of their contents to the tissues around when again 
squeezed. ‘Thus recurrent thrusts or impacts, acting on the 
body from without, aid in the nutrition of the parts on which 
they fall: producing, in some cases, a node upon the subjacent 
bone, as on the instep where a boot has pinched ; producing, 
in other cases, growth of the connective tissue, as in a bunion; 


THE OUTER TISSUES OF ANIMALS. 299 


and producing, more frequently, thickening of the epidermis.* 
It is no doubt true that the sensation which pressure causes, 
propagated to the spinal chord, and reflected thence through 
the vaso-motor nerve going to the spot, aids the process 
by exciting a wave of contraction along the minute arteries, 
thereby helping them to refill the capillaries the instant the 
pressure is taken off; and doubtless, as alleged, the excessive 
exudation that forms a blister when the intermittent com- 
pressions are violent and long-continued, is attributable to 
this reflex nervous action. But it is clear that the nervous 
action is secondary, and cannot of itself produce the effect ; 
for in the absence of intermittent pressure no exudation takes 
place, however acute and persistent the sensation may be. 
Continued pressure produces absorption instead of exudation. 

In animals therefore, as in plants, the external mechanical 
actions to be resisted, are themselves directly instrumental in 
working in the tissues they fall upon, the changes which fit 
those tissues to meet them. And it needs but to contemplate 
the process of thickening described, to see that it will go on 
until the shield produced suffices to protect the capillaries 
from excessive pressures—will go on, that is, until there is 
equilibrium between the outer and inner forces. 


§ 294. Dermal structures of another class are developed 
mainly, if not wholly, by the actions of external causes 
on species rather than on individuals. These are the 


* An inquiry into the causes of these differences of result, brings further 
evidence to light. The condition under which only the hypertrophy can 
arise, is that the pressure intermits sufficiently to allow the capillaries to 
refill frequently. The epidermis thickens where the pressures are habitually 
taken off so completely, that the capillari2s next the surface can refill, as in the 
hands. If we consider what happens where the instep is pressed by a tight 
boot, we shall see that the variations of pressure which occur in walking, do 
not suffice to relieve the quite superficial vessels and allow them to refill ; but 
in consequence of the slight mobility and elasticity of the tissues, the vessels 
at some distance beneath the surface are able to refill, and hence the thicken- 
ing occurs round them. 


300 PHYSIOLOGICAL DEVELOPMENT. 


various kinds of clothing—hairs, feathers, quills, scales, 
scutes. 

Readers who are unfamiliar with the extreme modifiability 
of organic structures, will be startled by the proposition that 
all of these—certainly all of them but the last, respecting 
which there may be doubts—are homologous parts. In- 
spection of a few cases makes this seemingly-incredible pro- 
position not simply credible but obviously true. A retrograde 
metamorphosis from feathers to appendages that are almost 
scale-like, is well seen in the coat of the Penguin. Carry 
the eye along the surface of one of these birds, and there is 
manifest a transition from the bird-hke covering to the fish- 
like covering—a transition so gradual that no place can be 
found where an appreciable break occurs. Less striking 
perhaps, but scarcely less significant, are the modifications 
‘through which we pass from feathers to hairs, on the surfaces 
of the Ostrich and the Cassowary. The skin of the Porcupine 
shows us hairs and quills united by a series of intermediate 
structures, differing from one another inappreciably. Even 
more remarkable is the extension of this alliance to certain 
other dermal structures. ‘It may be taken as certain, I 
think,” says Prof. Huxley, “that the scales, plates, and 
spines of all fishes are homologous organs; nor as less so 
that the tegumentary spines of the Plagiostomes are homo- 
logous with their teeth, and thence with the teeth of all 
vertebrata. Again, it appears to me indubitable that the 
teeth and the hairs are homologous organs.” 

The ultimate justification for classing these unlike parts as 
divergent modifications of the same thing, is the unity in 
their modes of development. Besides a linking together of 
them by intermediate structures, as above indicated, there is 
a linking together by their common origin. To quote again 
from Prof. Huxley’s essay on “Tegumentary Organs” :— 
“ The Hairs and Spines of mammals, the Feathers of birds, 
and the Integumentary Glands, agree in one essential point, 
that their development is preceded by that of an involution 


THE OUTER TISSUES OF ANIMALS. 301 


of the ecderon, within which they are formed, and by which 
the former are, at first, entirely enclosed.” And though the 
scales of fishes and the dermal plates of reptiles present diffi- 
culties, yet Prof. Huxley concludes that the course of their 
development is at first essentially the same. Some idea of it, 
and of the relations it proves among these structures, may be 
given thus :—Suppose a small pit to be formed on the pre- 
viously flat skin; and suppose that the growth anc casting 
off of horny cells which goes on over the skin in general, 
continues to go on at the usual rate over the depressed surface 
of this pit. Clearly the quantity of horny matter produced 
within this hollow, will be greater than that produced on a 
level portion of the skin subtending an equal area of the 
animal’s outside. Suppose such a pit to be deepened 
until it becomes a small sac. If the exfoliation goes on as 
before, the result will be that the horny matter, expelled, as 
it must be, through the mouth of the sac, which now bears 
a small proportion to the internal surface of the sac, will be 
large in quantity compared with that exfoliated from a 
portion of the skin equal in area to the mouth of the sac: 
there will be a conspicuous thrusting forth of horny matter. 
Suppose once more that the sac, instead of remaining simple, 
has its bottom pushed up into its interior, like the bottom of 
a beer-bottle—the introversion being carried so far that the 
introverted part reaches nearly to the external opening, and 
leaves scarcely any space between the introverted part and 
the walls of the sac. It is easy to see that the exfoliation 
continuing from the surface of the introverted part as well as 
from the inside of the sac generally, the horny matter cast 
off will form a double layer; and will come out of the sac 
in the shape of a tube having within its lower end the intro- 
verted part, as the core on which it is moulded, and from the 
apex of which is cast off the substance filling, less densely, 
its interior. The structure resulting will be what we know 
asa hair. Manifestly by progressive enlargement of the sac, 
aud further complication of that introverted part on which. 


302 PHYSIOLOGICAL DEVELOPMENT. 


the excreted substance is moulded, the protruding growth may 
be rendered larger and more involved, as we see it in quills 
and feathers. So that insensible steps, thus indicated in 
principle, carry us from the exfoliation of epidermis by a flat 
surface, to the exfoliation of it by a hollow simple sac, an 
introverted sac, and a sac further complicated ; each of which 
produces its modified kind of tegumentary appendage, 


§ 295. Among many other differentiations of the outer 
tissues, the most worthy to be noticed in the space that re- 
mains, are those by which organs of sense are formed. We 
will begin with the simplest and most closely allied to the 
foreguing. 

Every hair that is not too long or flexible to convey to its 
rooted end a strain put upon its free end, is a rudimentary 
tactual organ; as may be readily proved by touching one of 
those growing on the back of the hand. If, then, a creature 
has certain hairs so placed that they are habitually touched 
by the objects with which it deals, or amid which it moves, 
an advantage is likely to accrue if these hairs are modified 
in a way that enables them the better to transmit the im- 
pressions derived. Such modified hairs we have in the 
vibrisse, or, as they are commonly called, the ‘“ whiskers” 
possessed by Cats and feline animals generally, as well as by 
Seals and many Rodents. These hairs are long enough to 
reach objects at considerable distances ; they are so stiff that 
forces applied to their free ends, cause movements of their 
imbedded ends ; and the sacs containing their imbedded ends 
being well covered with nerve-fibres, these developed hairs 
serve as instruments of exploration. By constant use of them 
the animal learns to judge of the relative positions of objects 
past which, or towards which, it is moving. When stealthily 
approaching prey or stealthily escaping enemies, such aids to 
perception are obviously important: indeed their importance 
has been proved by the diminished power of self-guidance in 
the dark, that results from cutting them off. These, then, are 


THE OUTER TISSUES OF ANIMALS, 303 


dermal appendages originally serving the purpose of cloth- 
ing, but afterwards differentiated into sense-organs, 

That eyes are essentially dermal structures seems scarcely 
conceivable. Yet an examination of their rudimentary types, 
and of their genesis in creatures that have them well deve- 
loped, shows us that they really arise by successive modifica- 
tions of the double layer composing the integument. They 
make their first appearance among the simpler animals as 
specks of pigment, covered by portions of epidermis slightly 
convex and a little more transparent than that around it. 
Here their fundamental community of structure with the 
skin is easy to trace; and the formation of them by differen- 
tiation of it presents no difficulty. Not so far 
in advance of these as much to obscure the relationship, are 
the eyes which the Crustaceans possess. In every fish- 
monger’s shop we may see that the eyes of a Lobster are 
carried on pedicles ; and when the Lobster casts its shell, the 
outer coat of each eye, being continuous with the epidermis 
of its pedicle, is thrown off along with the rest of the exo- 
skeleton. This pedicle, which gives the name of “ stalk- 
eyed” Crustacea to a large group, is, strange as it may seem, 
a transformed limb. Otherwise shown by the homologies of 
the parts, this truth is made manifest by those transitional cases 
in which the original form of the limb is retained, and the 
transparent portion which serves as a visual organ is merely 
a prominent patch on its under surface, somewhat like a blister, 
spreading a little up the sides of the limb—an arrangement 
almost thrusting upon us the suspicion that an eye is a 
modified portion of the skin. That which the outer appear- 
ance suggests is proved by the structure within. Beneatn 
the transparent epidermic layer, there exists a group of eyes 
of the kind which we see in an insect; and these, according 
to a high authority, are inclosed in the dermal system. De- 
scribing the arrangement of the parts, M. Milne Edwards 
writes :—‘ But the most remarkable circumstance is, that the 
large cavity within which the whole of these parallel 


304 PHYSIOLOGICAL DEVELOPMENT. 


columns, every one of which is itself a perfect eye, are 
contained, is closed posteriorly by a membrane, which 
appears to be neither more nor less than the middle 
tegumentary membrane, pierced for the passage of the 
optic nerve; so that the ocular chamber at large 
results from the separation at a point of the two external 
layers of the general envelope.” Thus too is 
it, in the main, even with the highly-developed eyes of 
the Vertebrata. ‘‘ The three pairs of sensory organs apper- 
taining to the higher senses,” says Prof. Huxley—“ the nasal 
sacs, the eyes, and the ears—arise as simple ccecal involutions 
of the external integument of the head of the embryo. 
That such is the case, so far as the olfactory sacs are con- 
cerned, is obvious, and it is not difficult to observe that the 
lens and the anterior chamber of the eye are produced in a 
perfectly similar manner. Jt is not so easy to see that the 
labyrinth of the ear arises in this way, as the sac resulting 
from the involution of the integument is small, and remains 
open but a very short time. But I have so frequently veri- 
fied Huschke’s and Remak’s statement that it does so arise, 
that I entertain no doubt whatever of the fact. The outer 
ends of the olfactory sacs remain open, but those of the 
ocular and auditory sacs rapidly close up, and shut off their 
eontents from all direct communication with the exterior.” 
So that, marvellous as the fact appears, all that part of the 
eye which lies between its outer surface and the back of the 
crystalline lens, is formed in the same way as an ordinary hair- 
sac, and is composed of homologous parts. The interior coat is 
the epidermic layer, originally continuous with the surface of 
the skin; and only made discontinuous with it by closure of 
the sac at the point which is afterwards the centre of the 
cornea. This cornea, or front wall of the chamber thus shut 
off, is consequently composed of a doubled epidermic layer 
and an intermediate layer of the derma included in the fold 
of the integument. The crystalline lens, lying at the far side 
of this chamber, is simply a thickening of the epidermic layer 


THE OUTFR TISSUES OF ANIMALS. 305 


lining that part of the chamber—is developed from it in the 
same way that the substance of a hair is developed from the 
papilla at the bottom of its sac. The iris originates as an 
annular thrusting-in of the walls of this chamber in front of 
the crystalline lens; and between the two layers of the epi- 
dermic lining, thus folded, comes a portion of the derma in 
which muscular fibres eventually arise. Though the founda- 
tion of the part behind the crystalline lens is laid by a hollow 
diverticulum from the brain, which grows outwards to meet the 
inward-growing tegumentary sac, yet here, too, structures be- 
longing to the tegumentary system eventually predominate. 
For into this cul-de-sac proceeding from the nervous centre, 
there takes place a lateral growth of dermal tissue, which, in- 
troverting the wall of the sac, and presently filling the whole 
cavity of it, is at last shut off by the closure of the now 
doubled walls of the sac; and out of this intruding mass of 
dermal tissue the vitreous humour is formed, ‘That 1s to say, 
the eye considered as an optical apparatus is wholly produced 
by metamorphoses of the skin: the only parts of it not thus 
produced, being the membranes lying between the sclerotic 
and the vitreous humour, including those retinal structures 
formed in them. Allis tegumentary save that which has to 
appreciate the impressions which the modified integument 
concentrates upon it. 

Thus, as Prof. Huxley has somewhere pointed out, there 
is a substantial parallelism between all the sensory organs in 
their modes of development: as there is, too, between their 
modes of action. A vibrissa may be taken as their common 
type. Increased impressibility by an external stimulus, 
requires an increased peripheral expansion of the nervous 
system on which the stimulus may fall; and this is secured 
by an introvertion of the integument, forming a sac on the 
walls of which a nerve may ramify. That the more extended 
sensory area thus constituted may be acted upon, there 
requires some apparatus conveying to it from without the 


appropriate stimulus; and in the case of the vibrissa, this 
VOL. IL 20 


306 PHYSIOLOGICAL DEVELOPMENT. 


apparatus is the epidermic growth which, under the form of 
a hair, protrudes from the sac. And that the greatest 
sensitiveness may be obtained, the external action must be 
exaggerated or multiplied by the apparatus which conveys it 
to the recipient nerve ; as in the case of the vibrissa, it is by 
the development of a hair into an elastic lever, that trans- 
forms the slight force acting through considerable space on 
its exposed end, into a greater force acting through a smaller 
space at its rooted end. Similarly with the organs of the 
higher senses. In a rudimentary eye, we have but a slight 
peripheral expansion of a nerve to take cognizance of the 
impression ; and to concentrate the impression upon it, there 
is nothing beyond a thickening of the epidermis into a lens- 
shape. But the developed eye shows us a termination of the 
nerve greatly expanded and divided to receive the external 
stimulus. It shows us an introverted portion of the integu- 
ment containing the apparatus by which the external stimulus 
is conveyed to the recipient nerve. The structure developed 
in this sac not only conveys the stimulus, but also, like its 
homologue, concentrates it; aud in the one case as in the 
other, the structure which does this is an epidermic growth 
from the bottom of the sac. Even with the ear itis the same. 
Again we have an introverted portion of the integument, on 
the walls of which the nerve is distributed. The otolithes 
contained in the sac thus formed, are bodies which are set ir 
motion by the vibrations of the surrounding medium, and 
convey these vibrations in an exaggerated form to the nerves. 
And though it is not alleged that these otolithes are 
developed from the epidermic lining of the chamber, yet as, 
if not so developed, they are concretions from the contents of 
an epidermic sac, they must still be regarded as epidermic 
products. 

Whether these differentiations are due wholly to indirect 
equilibration, or whether direct equilibration has had a share 
in working them, are questions that must be left open. 
Possibly a short hair so placed on a mammal’s face as to be 


THE OUTER TISSUES OF ANIMALS. 307 


habitually touched, may, by conveying excitations to the 
nerves and vessels at its root, cause extra growth of the 
bulb and its appendages, and so the development of a vibrissa 
may be furthered. Possibly too, the light itself, to which the 
tissues of some inferior animals are everywhere sensitive, may 
aid in setting up certain of the modifications by which the 
nervous parts of visual organs are formed—producing, as it 
must, the most powerful effects at those points on the surface 
which the movements of the animal expose to the greatest 
and most frequent contrasts of light and shade; and propa- 
gating from those points currents of molecular change through 
the organism. But it seems clear that the complexities of 
the sensory organs are not thus explicable. They must have 
arisen by the natural selection of favourable variations. 


§ 296. A group of facts, serving to elucidate those put 
together in the several foregoing sections, has to be added. 
I have reserved this group to the last, partly because it is 
transitional—links the differentiations of the literally outer 
tissues with those of the truly inner tissues. Though physi- 
cally internal, the mucous coat of the alimentary canal has 
a guasi-externality from a physiological point of view. As 
was pointed out in the last chapter, the skin and the assimi- 
lating surface have this in common, that they come in direct 
contact with matters not belonging to the organism; and 
we saw that along with this community of relation to alien 
substances, there is a certain community of structure and de- 
velopment. The like holds with the linings of all internal 
cavities and canals that have external openings. 

The transition from the literally outer tissues to those 
tissues that are intermediate between them and the truly 
inner tissues, is visible at all the orifices of the body ; where 
skin and mucous membrane are continuous, and the one 
passes insensibly into the other. This visible continuity is 
not simply associated with a great degree of morphological 


continuity, but also with a great degree of physiological con- 
20 # 


308 PHYSIOLOGICAL DEVELOPMENT. 


tunuity. That is to say, these literally outer and guasi-outer 
layers are capable of rapidly assuming one another’s struc- 
tures and functions when subject to one another’s conditions. 
Mucous surfaces, normally kept covered, become skin-like if 
exposed to the air; but resume more or less fully their 
normal characters when restored to their normal positions. 
These are truths familiar to pathologists. They continually 
meet with proofs that permanent eversion of the mucous 
membrane, even where it is by prolapse of a part deeply 
seated within the body, is followed by an adaptation eventu- 
ally almost complete: originally moist, tender to the touch, 
and irritated by the ‘air, the surface gradually becomes 
covered with a thick, dry cuticle; and is then scarcely more 
sensitive than ordinary integument. 

Whether this equilibration between new outer forces and 
reactive inner forces, which is thus directly produced in in- 
dividuals, is similarly produced in races, must remain as a 
question not to be answered in a positive way. On the one 
hand, we have the fact that among the higher animals there 
are cases of quasi-outer tissues which are in one species 
habitually ensheathed, while in another species they are not 
ensheathed ; and that these two tissues, though unquestion- 
ably homologous, differ as much as skin and mucous mem- 
brane differ. On the other hand, there are certain analogous 
changes of surface, as on the abdomen of the Hermit-Crab, 
which give warrant to the supposition that survival of the 
fittest is the chief agent in establishing such differentiations ; 
since the abdomen of a Hermit-Crab, bathed by water within 
the shell it occupies, is not exposed to physical conditions 
that directly tend to differentiate its surface from the surface 
of the thorax. But though in cases like this last, we must 
assign the result to the natural selection of variations arising 
incidentally ; we may I think legitimately assign the result 
to the immediate action of changed conditions where, as in 
cases like the first, we see these producing in the individual, 
effects of the kinds observed in the race. 


THE OUTER TISSUES OF ANIMALS. 309 


However this may be, the force of the general argument 
remains the same. In these exchanges of structure and 
function between the outer and quwasi-outer tissues, we get 
undeniable proof that they are easily differentiable. And 
seeing this, we are enabled the more clearly to see how there 
have, in course of time, arisen those extreme and multi- 
tudinous differentiations of the outer tissues that have been 
glanced at. 


QHAPTER VIII. 


DIFFERENTIATIONS AMONG THE INNER TISSUES OF 
ANIMALS, 


§ 297. The change from the outside of the lips to their 
inside, introduces us to a new series of interesting and 
instructive facts, joining on to those with which the last 
chapter closed. They concern the differentiations of those 
coats of the alimentary canal, which, as we have seen, are 
physiologically outer, though physically inner. 

These coats are greatly modified at different parts; and 
their modifications vary greatly in different animals. In 
the lower types, where they compose a simple tube, running 
from end to end of the body, they are almost uniform in their 
histological characters; but on ascending from these types, 
we find them presenting an increasing variety of minute 
structures between their two ends. The argument will be 
adequately enforced if we limit ourselves to the leading 
modifications they display in some of the higher animals, 

The successive parts of the alimentary canal are so placed 
with respect to its contents, that the physical and chemical 
changes undergone by its contents while passing from one 
end to the other, inevitably tend to transform its originally 
homogeneous surface into a heterogeneous surface. Clearly, 
the effect produced on the food at any part of the canal by — 
trituration, by adding a secretion, or by absorbing its nutri- 
tive matters, implies the delivery of the food into the next 
part of the canal in a state more or less unlike its previous 


ea ee, <i 


THE INNER TISSUES OF ANIMALS. 3ll 


states—implies that the surface with which it now comes in 
contact is differently affected by it from the preceding sur- 
faces—implies, that is,a differentiating action. To use con- 
crete language ;—food that is broken down in the mouth acts 
on the csophagus and stomach in a way unlike that which 
it would have done had it been swallowed whole; the masti- 
cated food, to which certain solvents or ferments are added, 
becomes to the intestine a different substance from that which 
it must have otherwise been; and the altered food, resolved 
by these additions into its proximate principles, cannot have 
those proximate principles absorbed in the next part of the 
intestine, without the remoter parts being affected as they 
would not have been in the absence of absorption. It is true 
that in developed alimentary canals, such as the reasoning 
here tacitly assumes, these marked successive differentiations 
of the food are themselves the results of pre-established 
differentiations in the successive parts of the canal. But it is 
also true that actions and reactions like those here so definitely 
marked, must go on indefinitely in an undeveloped alimentary 
canal. If the food ischanged at all in the course of its transit, 
which it must be if the creature is to live by it, then it 
cannot but act dissimilarly on the successive tracts of the 
alimentary canal, and cannot but be dissimilarly reacted on 
by them. Inevitably, therefore, the uniformity of the surface 
must lapse into greater or less multiformity : the differentia- 
tion of each part tending ever to initiate differentiations of 
other parts. 

Not, indeed, that the implied process of direct equilibra- 
tion can be regarded as the sole process. Indirect equilibra- 
tion aids; and, doubtless, there are some of the modifications 
which only indirect equilibration can accomplish. But we 
have here one unquestionable cause—a cause that is known 
to work in individuals, changes of the kind alleged. Where 
for instance, cancerous disease of the cesophagus so narrows 
the passage into the stomach as to prevent easy descent of 
the food, the esophagus above the obstruction becomes 


312 PHYSIOLOGICAL DEVELOPMENT. 


enlarged into a kind of pouch; and the inner surface of this 
pouch begins to secrete juices that produce in the food a kind 
of rude digestion. Again, stricture of the intestine, when it 
arises gradually, is followed by hypertrophy of the muscular 
coat of the intestine above the constricted part: the ordinary 
peristaltic movements being insufficient to force the food 
forwards, and the lodged food serving as a constant stimulus 
to contraction, the muscular fibres, habitually more exercised, 
become more bulky. The deduction from general principles 
being thus inductively enforced, we cannot, I think, resist 
the conclusion that the direct actions and reactions between 
the food and the alimentary canal have been largely instru- 
mental in establishing the contrasts among its parts. And 
we shall hold this view with the more confidence on observy- 
ing how satisfactorily, in pursuance of it, we are enabled to 
explain one of the most striking of these differentiations, 
which we will take as a type of the class. 

The gizzard of a bird is an expanded portion of the alimen - 
tary canal, specially fitted to give the food that trituration 
which the toothless mouth of the bird cannot give. Besides 
having a greatly-developed muscular coat, this grinding- 
chamber is lined with a thick, hard cuticle, capable of 
bearing the friction of the pebbles swallowed to serve as 
grind-stones. This differentiation of the mucous coat into a 
ridged and tubercled layer of horny matter—a differentiation 
which, in the analogous organs of certain Mollusca, is carried. 
to the extent of producing from this membrane bony plates, 
and even teeth—varies in birds of different kinds, according 
to their food. It is moderate in birds that feed on flesh and 
fish, and extreme in granivorous birds and others that live 
on hard substances. How does this immense modification of 
the alimentary canal originate ? In the stomach 
of a mammal, the macerating and solvent actions are united 
with that triturating action which finishes what the teeth 
have mainly done; but in the bird, unable to masticate, these 
internal functions are specialized, and while the crop is the 


THE INNER TISSUES OF ANIMALS. 313 


macerating chamber, the gizzard becomes a chamber adapted 
to triturate more effectually. This adaptation requires simply 
an exaggeration of certain structures and actions which 
characterize stomachs in general, and, in a less degree, 
alimentary canals throughout their whole lengths. The 
massive muscles of the gizzard are simply extreme develop- 
ments of the muscular tunic, which is already considerably 
developed over the stomach, and incloses also the cesophagus 
and the intestine. The indurated lining of the gizzard, 
thickened into horny buttons at the places of severest pres- 
sure, is nothing more than a greatly strengthened and 
modified epithelium. And the grinding action of the gizzard 
is but a specialized form of that rhythmical contraction by 
which an ordinary stomach kneads the contained food, and 
which in the cesophagus effects the act of swallowing, while 
in the intestine it becomes the peristaltic motion. Allied as 
the gizzard thus clearly is in structure and action to the 
stomach and alimentary canal in general; and capable of 
being gradually differentiated from a stomach where a grow- 
ing habit of swallowing food unmasticated entails more 
trituration to be performed before the food passes the pylorus; 
the question is—Does this change of structure arise by direct 
adaptation? There is warrant for the belief that it does. 
Besides such collateral evidence as that mucous membrane 
becomes horny on the toothless gums of old people, when 
subject to continual rough usage, and that the muscular coat 
of the intestine thickens where unusual activity is demanded 
of it, we have the direct evidence of experiment. Hunter 
habituated a sea-gull to feed upon grain, and found that the 
lining of its gizzard became hardened, while the gizzard- 
muscles doubled in thickness. <A like change in the diet of 
a kite was followed by like results. Clearly, if differentiations 
so produced in the individuals of a race under changed habits, 
are in any degree inheritable, a structure like a gizzard will 
originate through the direct actions and reactions between 
the food and the alimentary canal. 


314 PHYSIOLOGICAL DEVELOPMENT. 


Another case—a very interesting one, somewhat allied to 
this—is presented by the ruminating animals. Here several 
dilatations of the alimentary canal precede the true stomach ; 
and in these, large quantities of unmasticated food are stored, 
to be afterwards returned to the mouth and masticated at 
leisure. What conditions have made this specialization 
advantageous ? and by what process has it been established ? 
To both these questions the facts indicate answers which are 
not unsatisfactory. Creatures that obtain their 
food very irregularly— now having more than they can 
consume, and now being for long periods without any—must, 
in the first place, be apt, when very hungry, to eat to the 
extreme limits of their capacities; and must, in the second 
place, profit by peculiarities which enable them to compensate 
themselves for long fasts, past and future. A perch which, 
when its stomach is full of young frogs, goes on filling its 
esophagus also; or a trout which, rising to the fisherman’s 
fly, proves when taken off the hook to be full of worms and 
insect-larvee up to the very mouth, gains by its ability to take 
in such unusual supplies of food when it meets with them — 
obviously thrives better than it would do could it never eat 
more than a stomachful. That this ability to feed greatly in 
excess of immediate requirement, is one that varies in indi- 
viduals of the same race, we see in the marked contrast 
between our own powers in this respect, and the powers of 
uncivilized men; whose fasting and gorging are to us so 
astonishing. Carrying with us these considerations, we shall 
not be surprised at finding dilatations of the esophagus in 
vultures and eagles, which get their prey at long intervals 
in large masses ; and we may naturally look for them too in 
birds like pigeons, which, coming in flocks upon occasional 
supplies of grain, individually profit by devouring the 
greatest quantity in a giver time. Now where the trituration 
of the food is, as in these cases, carried on in a lower part of 
the alimentary canal, nothing further is required than the 
storing-chamber; but for a mammal, having its grinding 


THE INNER TISSUES OF ANIMALS. 315 


apparatus in its mouth, to gain by the habit of hurriedly 
swallowing unmasticated food, it must also have the habit of 
regurgitating the food for subsequent mastication. ‘This 
correlation of habits with their answering structures, may, as 
we shall see, arise in a very simple way. The 
starting point of the explanation is a familiar fact—the fact 
that indigestion, often resulting from excess of food, is apt to 
cause that reversed peristaltic action known as vomiting. 
From this we pass to the fact, aiso within the experience of 
most persons, that during slight indigestion the stomach some- 
times quietly regurgitates a small part of its contents as far 
as the back of the mouth—giving an unpleasant acquaintance 
with the taste of the gastric juices. Exceptional facts of the 
same class help the argument a step further. ‘There are 
certain individuals who are capable of returning, at will, a 
greater or smaller portion of the contents of the digesting 
stomach into the cavity of the mouth. * * * In some of these 
cases, the expulsion of the food has required a violent effort. In 
the majority, it has been easily evoked or suppressed. While 
in others, it has been almost uncontrollable; or its non- 
occurrence at the habitual time has been followed by a 
painful feeling of fulness, or by the act of vomiting.” 
Here then we have a certain physiological action, occa- 
sionally happening in most persons and in some developed 
into a habit more or less pronounced : indigestion being the 
habitual antecedent. Suppose then that gregarious 
animals, living on innutritive food such as grass, are subject to 
a like physiological action, and are capable of like varia- 
tions in the degree of it. What will naturally happen ? 
They wander in herds, now over places where food is scarce 
and now coming to places where it is abundant. Some mas- 
ticate their food completely before swallowing it; while some 
masticate it incompletely. If an oasis, presently bared by 
their grazing, has not supplied the whole herd a full meal, 
then the individuals which masticate completely will have 
had less than those which masticate incompletely—will not 


d16 PHYSIOLOGICAL DEVELOPMENT. 


have had enough. Those which masticate incompletely and 
distend their stomachs with food difficult to digest, will be 
liable to these regurgitations; but if they re-masticate what 
is thus returned to the mouth (and we know that animals 
often eat again what they have vomited), then the extra 
quantity of tood taken, eventually made digestible, will yield 
them more nourishment than is obtained by those which 
masticate completely at first. The habit initiated in this 
natural way, and aiding survival when food is scarce, 
will be apt to cause modifications of the alimentary 
canal. We know that dilatations of canals readily arise 
under habitual distensions. We know that canals habitu- 
ally distended become gradually more tolerant of the 
contained masses that at first irritated them. And we know 
that there commonly take place adaptive modifications of their 
surfaces. Hence if a habit of this kind and the structural 
changes resulting from it, are in any degree inheritable, it is 
clear that, increasing in successive generations, both imme- 
diately by the cumulative effect of repetitions and mediately 
by survival of the individuals in which they are most decided, 
_they may go on until they end in the peculiarities which 
Ruminants display. 


§ 298. There are structures belonging to the same group 
which cannot, however, be accounted for in this way. They 
are the organs that secrete special products facilitating 
digestion—the liver, pancreas, and various smaller glands. 
All these appendages of the alimentary canal, large and 
independent as some of them seem, really arise by differen- 
tiations from its coats. ‘The primordial liver, as we see it in 
a simple animal such as the Planaria, consists of nothing 
more than bile-cells scattered along a tract of the intestinal 
surface. Accumulation of these bile-cells is accompanied by 
increased growth of the surface which bears them—a growth 
which at first takes the form of a cul-de-sac, having an outside 
that projects from the intestine into the peri-visceral cavity. 


THE INNER TISSUES OF ANIMAIS. dl7 


As the mass of bile-cells becomes greater, there arise se- 
condary lateral cavities opening into the primary one, and 
through it into the intestine; until eventually these cavities 
with their coatings of bile-cells, become ramifying ducts dis- 
tributed through the solid mass we know as a liver. How is 
this differentiation caused P 

Before attempting any answer to this question, it is requisite 
to inquire the nature of bile. Is that which the liver throws 
into the intestines a waste product of the organic actions? or 
is it a secretion aiding digestion P or is it mixture of these ? 
Modern investigations imply that it is most likely the last. 
The liver is found to have a compound function. Bernard 
has proved to the satisfaction of physiologists, that there goes 
on in it a formation of glycogen—a substance that is trans- 
formed into sugar before it leaves the liver and is afterwards 
carried away by the blood to eventually disappear in the lungs. 
It is also shown, experimentally, that there are generated in 
the liver certain biliary acids; and by the aid either of 
these or of some other compounds, it is clear that bile 
renders certain materials more absorbable: its effect on 
fat is demonstrable out of the body; and the greatly 
diminished absorption of fat from the food when the 
discharge of bile into the intestine is prevented, is probably 
one of the causes of that pining away that results. But while 
recognizing the fact that the bile consists in part of a 
solvent, or solvents, aiding digestion, there is abundant 
evidence that one element of it is an effete product ; and 
probably this is the primary element. The yellow-green 
substance called biliverdine, which gives its colour to bile, is 
found in the blood before it reaches the liver; which is not 
the case with the glycogen or the biliary acids. ‘‘ As soon as 
the biliary secretion is in abeyance,” says Dr. Harley, the 
most recent authority on the subject, “ biliverdine accumu- 
lates in the blood (until the serum is as it were completely 
saturated with the pigment), from which it exudes and stains 
the tissues, and produces the colour we term jaundice ;” 


318 PHYSIOLOGICAL DEVELOPMENT. 


* * * “the urine assumes a saffron tint in consequence of 
the elimination of the colouring matter by the kidneys ;”’ and 
afterwards ‘the sweat, the milk, the tears, the sputa” become 
yellow. We have clear proof, then, that biliverdine is an 
excrementitious matter, which, if not got rid of through the 
liver, makes its way out, to some extent, through other or- 
gans, producing in them more or less derangement—itching 
of the skin, and sometimes, in the kidneys, a secondary 
disease. That of the bile discharged into the intestine, only 
some components are re-absorbed, is demonstrated by the fact 
that when injected into the blood, bile destroys life in less 
than twenty-four hours ; and that biliverdine is not among 
the re-absorbed components, is shown both by the persistence 
of the colour which it gives to the substances in the intestine, 
and by the absence of that jaundice which, if re-absorbed, 
it would produce. Hence we are warranted in classing bili- 
verdine as a waste product. And considering that the bile- 
cells, where they first make their appearance among animals, 
are distinguished by the colour ascribable to this substance, 
we may fairly infer that the excretion of biliverdine is the 
original function of the liver. 

One further preliminary is requisite. We must for a 
moment return to those physico-chemical data, set down in the 
first chapter of this work (§§ 7—8.) We there saw that 
the complex and large-atomed colloids which mainly compose 
living organic matter, have extremely little molecular mo- 
bility ; and, consequently, extremely little power of diffusing 
themselves. Whereas we saw not only that those absorbed 
matters, gaseous and liquid, which further the decomposition 
of living organic matter, have very high diffusibilities ; but 
also that the products of the decomposition are much more 
diffusible than the components of living organic matter. Ane. 
we saw that, as a consequence of this, the tissues give ready 
entrance to the substances that decompose them, and ready 
exit to the substances into which they are decomposed. Hence 
it follows that, primarily, the escape of effete matters from the 


THE INNER TISSUES OF ANIMALS. 319 


organism, is a physical action parallel to that which goes on 
among mixed colloids and crystalloids that are dead or even 
inorganic. Hxcretion is simply a specialized form of this 
spontaneous action ; and what we have to inquire is,—how the 
specialization arises. 

Two causes conspire to establish it. The first is that these 
products of decomposition are diffusible in widely different 
degrees. While the carbonic acid and water permeate the 
tissues with ease in all directions, and escape more or less 
from all the exposed surfaces, urea, and other waste substances 
incapable of being vaporized, cannot escape thus readily. 
The second is that the different parts of the organism, being 
subject to different physical conditions, are from the outset 
sure severally to favour the exit of these various products of 
decomposition in various degrees. How these causes must 
have co-operated in localizing the excretions, we shall see on 
remembering how they now co-operate in localizing the sepa- 
ration of morbid materials. The characteristic substances of 
gout and rheumatism have their habitual places of deposit. 
Tuberculous matter, though it may be present in various 
organs, gravitates towards some much more than towards 
others. Certain products of disease are habitually got rid of 
by the skin, instead of collecting internally. Mostly, these 
have special parts of the skin which they affect rather than 
the rest; and there are those which, by breaking out sym- 
metrically on the two sides of the body, show how definitely 
the places of their excretion are determined by certain favour- 
ing conditions, which corresponding parts may be presumed 
to furnish in equal degrees. Further, it is to be observed 
of these morbid substances circulating in the blood, that 
having once commenced segregating at particular places, 
they tend to continue segregating at those places. As- 
suming, then, as we may fairly do, that this localization 
of excretion, which we see continually commencing afresh 
with morbid matters, has always gone on with the matters 
produced by the waste of the tissues, let us take a further 


O20 PHYSIOLOGICAL DEVELOPMENT. 


step, and ask how localizations become fixed. Other things 
equal, that which from its physical conditions is a place of 
least resistance to the exit of an effete product, will tend to 
become established as the place of excretion ; since the rapid 
exit of an effete product will profit the organism. Other 
things equal, a place at which the excreted matter produces 
least detrimental effect will become the established place. If 
at any point the excreted matter produces a beneficial effect, 
then, other things equal, natural selection will determine it 
to this point. And if facility of escape anywhere goes along 
with utilization of the escaping substance, then, other things 
equal, the excretion will be there localized by survival of the 
fittest. 

Such being the conditions of the problem, let us ask what 
will happen with the lining membrane of the alimentary 
canal. This, physiologically considered, is an external sur- 
face; and matters thrown off from it make their way out of 
the body. It isalso a surface along which is moving the food 
to be digested. Now, among the various waste products 
continually escaping from the living tissues, some of the 
more complex ones, not very stable in composition, are likely, 
if added to the food, to set up changes init. Such changes 
may either aid or hinder the preparation of the food for 
absorption. If an effete matter, making its exit through the 
wall of the intestine, hinders the digestive process, the 
enfeeblement and disappearance of individuals in which this 
happens, will prevent the intestine from becoming the esta- 
blished place for its exit. While if it aids the digestive 
process, the intestine will, for converse reasons, become more 
and more the place to which its exit is limited. Equally 
manifest is it that if there is one part of this alimentary canal 
at which, more than at any other part, the favourable effect 
results, this will become the place of excretion. If 
from this general statement we pass to the special case 
before us, we find our daa to be these :—The substance to be 
excreted, biliverdine, a waste prcduct of the organic actions, 


THE INNER TISSUES OF ANIMALS. 321 


is, as jaundice shows us, capable of escaping out of the body 
through ail its surfaces, even in so differentiated a type as the 
highest mammal; and in the undifferentiated types we may 
infer that the facility of escape is nearly the same through 
all the surfaces. For the gradual localization of its escape 
ata particular part of the intestinal surface, it is requisite 
only that either some disadvantage consequent on its escape 
elsewhere should be avoided, or some advantage due to its 
effect on digestion should be gained; and this advantage 
may be either direct or indirect. It is not necessary that 
the biliverdine should itself act on the food: it is enough if 
it aids in the elaboration of other matters, either nutritive or 
solvent. If its presence causes or furthers the formation of 
glycogen from other components of the blood; or if it sets up 
the complex reactions which generate the biliary acids; these 
effects will suffice to establish, as the place of its excretion, 
the place where these products are useful. And once this 
place of excretion having been established, the development 
of a liver is simply a question of time and natural selection. 

Whether in this case, as well as in the cases of the exclu- 
sively secreting glands formed along the alimentary canal (to 
which a modification of the foregoing argument is applicable), 
any tendency to localization results from the immediate action 
of the local conditions, is an interesting question. It is 
possible that the contrasts between the intra-vascular and 
extra-vascular liquids at these places may be a factor in the 
differentiation, as in a case already dealt with. (§ 292.) 
But this possibility must be left undiscussed. 


§ 299. A differentiation of another order occurring in the 
alimentary canal, is that by which a part of it is developed 
into a lateral chamber or chambers, through which carbonic 
acid exhales and oxygen is absorbed. Comparative anatomy 
and embryology unite in showing that a lung is formed, just 
as a liver or other appendage of the alimentary canal is 
formed, by the growth of a hollow bud into the peri-visceral 


VOL. IL 21 


322 PHYSIOLOGICAL DEVELOPMENT. 


cavity, or space between the alimentary canal and the wall of 
the body. The interior of this bud is simply a cud-de-sae of 
the alimentary canal, with the mucous lining of which its 
own mucous lining is continuous. And the development of 
this cul-de-sac into an air-chamber, simple or compound, is 
merely a great extension of area in the internal surface of 
the cul-de-suc, along with that specialization which fits it 
for excreting and absorbing substances different from those 
which other parts of the mucous surface excrete and 
abscrb. These lateral air-chambers, universal 
among the higher Vertebrata and very general among the 
lower, and everywhere attached to the alimentary canal 
between the mouth and the stomach, have not in all cases the 
respiratory function. In most fishes that have them they 
are what we know as swim-bladders. In some fishes tke 
cavities of these swim-bladders are completely shut off from 
the alimentary canal: nevertheless showing, by the communi- 
cations which they have with it during the embryonic stages, 
that they are originally diverticula from it. In other fishes 
there is a permanent ductus pneumaticus, uniting the cavity 
of the swim-bladder with that of the gullet—the function, 
however, being still not respiratory in an appreciable degree, 
if at all. But in certain still extant representatives of the 
sauroid fishes, as the Lepidosteus, the air-bladder is ‘‘ divided 
into two sacs that possess a cellular structure,” and ‘the 
trachea which proceeds from it opens high-up in the throat, 
and is surrounded with a glottis.’ In the Amphibia the 
corresponding organs are chambers over the surfaces of which 
there are saccular depressions, indicating a transition towards 
the air-cells characterizing lungs; and accompanying this 
advance we see, as in the common Triton, the habit of coming 
up to the surface and taking down a fresh supply of air in 
place of that discharged. 

How are the internal air-chambers, respiratory or non- 
respiratory, developed? Upwards from the amphibian stage, 
in which they are partially refilled at long intervals, there is 


THE INNER TISSUES OF ANIMALS. 320 


no difficulty in understanding how, by infinitesimal steps, 
they pass into complex and ever-moving lungs. But 
how is the differentiation that produces them initiated ? 
How comes a portion of the internal surface to be specialized 
for converse with a medium to which it is not naturally 
exposed? The problem appears a difficult one; but there is 
a not unsatisfactory solution of it. 

When many gold-fish are kept in a Saal aquarium, as 
with thoughtless cruelty they frequently are, they swim 
close to the surface, so as to breathe that water which is from 
instant to instant absorbing fresh oxygen. In doing this 
they often put their mouths partly above the surface, so that 
in closing them they take in bubbles of air; and sometimes 
they may be seen to continue doing this—the relief due to 
the slight extra aération of blood so secured, being the 
stimulus to continue. Air thus taken in may be detained. 
If a fish that has taken in a bubble turns its head down- 
wards, the bubble will ascend to the back of its month, and 
there lodge; and coming within reach of the contractions of 
the cesophagus, it may be swallowed. If, then, among fish 
thus naturally led upon occasion to take in air-bubbles, there 
are any having slight differences in the alimentary canal that 
facilitate lodgment of the air, or slight nervous differences 
such as in human beings cause an accidental action to be- 
come “‘a trick,” it must happen that if an advantage accrues 
from the habitual detention of air-bubbles, those individuals 
most apt to detain them, will, other things equal, be more 
likely than the rest to survive; and by the survival of 
descendants inheriting their peculiarities in the greatest 
degrees, and increasing them, an established structure and an 
established habit may arise. And that they do in some 
way arise we have proof: the common Loach is well known 
to swallow air, which it afterwards discharges loaded with 
carbonic acid. 

From air thus swallowed the advantages that may be 


derived are of two kinds. In the first place, the fish is made 
21% 


324 PHYSIOLOGICAL DEVELOPMENT. 


specifically lighter, and the muscular effort needed to keep it 
from sinking is diminished—or, indeed, if the bubble is of 
the right size, is altogether saved. The contrast between the 
movements of a Goby, which, after swimming up towards the 
surface falls rapidly to the bottom on ceasing its exertions, 
and the movements of a Trout, which remains suspended just 
balancing itself by slight undulations of its fins, shows how 
great an economy results from an internal float, to fishes which 
seek their food in mid-water or at the surface. Hence the 
habit of swallowing air having been initiated in the way 
described, we see why natural selection will, in certain fishes, 
aid modifications of the alimentary canal favouring its 
lodgment—modifications constituting air-sacs. ein 
the second place, while from air thus lodged in air-sacs thus 
developed, the advantage will be that of flotation only if the 
air is infrequently changed or never changed ; the advantage 
will be that of supplementary respiration if the air-sacs are 
from time to time partially emptied and refilled. The re- 
quirements of the animal will determine which of the two 
functions predominates. Let us glance at the different sets 
of conditions under which these divergent modifications 
may be expected to arise. 

The respiratory development is not likely to take place in 
fishes that inhabit seas or rivers in which the supply of 
aérated water never fails: there is no obvious reason why 
the established branchial respiration should be replaced by a 
pulmonic respiration. Indeed, if a fish’s branchial respiration 
is adequate to its needs, a loss would result from the effort of 
coming to the surface for air; especially during those first 
stages of pulmonic development when the extra aération 
achieved was but small. Hence in fishes so circumstanced, 
the air-chambers arising in the way described would naturally 
become specialized mainly or wholly into floats. Their con- 
tained air being infrequently changed, no advantage would 
arise from the development of vascular plexuses over their 
surfaces ; nothing would be gained by keeping open the com- 


THE INNER TISSUES OF ANIMALS. 325 


munication between them and the alimentary canal; and 
there might thus eventually result closed chambers the 
gaseous contents of which, instead of being obtained from 
without, were secreted from their walls, as gases often are 
from mucous membranes. Contrariwise, aquatic 
vertebrata in which the swallowing of air-bubbles, becoming 
habitual, had led to the formation of sacs that lodged the 
bubbles ; and which continued to inhabit waters not always 
supplying them with sufficient oxygen; might be expected 
to have the sacs further developed, and the practice of chang- 
ing the contained air made regular, if either of two advan- 
tages resulted—either the advantage of being able to live in 
old habitats that had become untenable without this modifi- 
cation, or the advantage of being able to occupy new habitats. 
Now it is just where these advantages are gained that we see 
the pulmonic respiration coming in aid of the branchial 
respiration, and in various degrees replacing it. Shallow 
waters. are liable to three changes which conspire: to make 
this supplementary respiration beneficial. The summer’s.sun 
heats them, and raising the temperatures of the animals they 
contain, accelerates the circulation in these animals, exalts 
their functional activities, increases the production of car- 
bonic acid, and thus makes aération of the blood more needful 
than usual. Meanwhile the heated water, instead of yielding 
to the highly carbonized blood brought to the branchiz the 
usual quantity of oxygen, yields less than usual; for as the heat 
of the water increases, the quantity of air it contains diminishes. 
And this greater demand for oxygen joined with smaller 
supply, pushed to an extreme where the water is. nearly all 
evaporated, is at last still more intensely felt in consequence 
of the excess of carbonic acid discharged by the numerous 
creatures congregated in the muddy puddles that remain. 
Here, then, it is, that the habit of taking in air-bubbles is 
likely to become established, and the organs for utilizing them 
developed ; and here it is, accordingly, that we find all stages 
of the transition to aérial respiration. The Loach before- 


326 PHYSIOLOGICAL DEVELOPMENT. 


mentioned, which swallows air, frequents small waters liable — 
to be considerably warmed ; and the Cuchia, an anomalous 
eel-shaped fish, which has vascular air-sacs opening out at the 
back of the mouth, ‘‘is generally found lurking in holes 
and crevices, on the muddy banks of marshes or slow-moving 
rivers.” Still more significant is the fact that the Lepidosiren, 
or ‘‘mud-fish” as it is called from its habits, is the only true 
fish that has lungs. But it is among the Amphibia that we see 
most conspicuously this relation between the development 
of air-breathing organs, and the peculiarities of the habitats. 
Pools, more or less dissipated annually, and so rendered unin- 
habitable by most fishes, are very generally peopled by these 
transitional types. Just as we see, too, that in various 
climates and in various kinds of shallow waters, the supple- 
mentary aérial respiration is needful in different degrees ; so 
do we find among the Amphibia many stages in the substi- 
tution of the one respiration for the other. The facts, then, 
are such as give to the hypothesis a wraisemblance greater than 
could have been expected. 

The relative effects of direct and indirect equilibration in 
establishing this further heterogeneity, must, as in many other 
cases, remain undecided. The habit of taking in bubbles is 
scarcely interpretable as a result of spontaneous variation : we 
must regard it as arising accidentally during the effort to 
obtain the most aérated water; as being persevered in 
because of the relief obtained ; and as growing by repetition 
into a tendency bequeathed to offspring, and by them, or 
some of them, increased and transmitted. The formation of 
the first slight modifications of the alimentary canal favouring 
the lodgment of bubbles, is not to be thus explained. Some 
favourable variation in the shape of the passage must here 
have been the initial step. But the gradual increase of this 
structural modification by the survival of individuals in which 
it is carried furthest, will, I think, be all along aided by 
immediate adaptation. The part of the alimentary canal 
previously kept from the air, but now habitually in contact 


THE INNER TISSUES OF ANIMALS, yey 


with the air, must be in some degree modified by the 
action of the air; and the directly-produced modification, 
increasing in the individual and in successive individuals, 
cannot cease until there isa complete balance between the 
actions of the changed agency and the changed tissue. It is 
indeed probable that the growth as well as the differentiation 
of the pulmonic surface, when once commenced, will be 
furthered by the direct process. The reasoning before 
used in the case of branchiew (§ 292) applies in the case 
of lungs. If exchange between the plasma in the blood- 
vessels and the plasma in the tissues surrounding them, 
goes on with a rapidity that becomes greater where the 
difference between them becomes greater; if, consequently, 
at some place where the carbonized plasma inside the 
blood-vessels is brought close to an unusually decarbonized 
or much oxygenated plasma outside of the blood-vessels, the 
exchange of these liquids becomes unusually active ; if, as a 
result, the circulation in the part is augmented ; then it is to 
be inferred that the extra nutrition will cause extra growth. 
The surface of the rudimentary lung will increase in area so 
long as the capillary osmose 1s much greater than in other 
parts of the body; and it will continue to be greater until, 
by the extension of the aérating surface, the respiratory 
exchange has been rendered so efficient as to bring down the 
contrast between the intra-vascular and extra-vascular liquids 
to a level with the contrasts between the intra-vascular and 
extra-vascular liquids in other organs. That is to say, the 
growth which this direct action produces, will go on until the 
functional efficiency of the lungs is in equilibrium with the 
functional efficiencies of other parts throughout the organism. 


§ 800. We come now to differentiations among the truly 
inner tissues—the tissues which have direct converse neither 
with the environment nor with the foreign substances taken 
into the organism from the environment. These, speaking 
broadly, are the tissues which lie between the double layer 


028 PHYSIOLOGICAL DEVELOPMENT. 

forming the integument with its appendages, and the double 
layer forming the alimentary canal with its diverticula. We 
will take first the differentiation which produces the vascular 
system. 

Certain forces producing and aiding distribution of liquids in 
animals, come into play before any vascular system exists ; 
and continue to further circulation after the development of 
a vascular system. The first of these is osmotic exchange, 
acting locally and having an indirect general action; the 
second is osmotic distension, acting generally and having an 
indirect local action; the third is local variation of pressure 
which movement of the body throws on the tissues and their 
contained liquids. A few words are needed in elucidation of 
each. If in any creature, however simple, different 
changes are going on in parts that are differently conditioned 
—if, as in a Hydra, one surface is exposed to the surrounding 
medium while the other surface is exposed to dissolved food ; 
then between the unlike liquids which the dissimilarly-placed 
parts contain, osmotic currents must arise; and a movement 
of liquid through the intermediate tissue must go on as long 
as an unlikeness between the liquids is kept up. This primary 
cause of re-distribution remains one of the causes of re-distri- 
bution in every more-developed organism: the passage of 
matters into and out of the capillaries is everywhere thus 
set up. And obviously in producing these local currents, 
osmose must also indirectly produce general currents, or aid 
them if otherwise produced. Osmose, however, still 
further aids circulation by the liquid pressure which it esta- 
blishes throughout the organism. More marked than the 
contrasts between the liquids in some parts and those in 
other parts, is the contrast between the whole mass of 
liquid in the animal and the liquid bathing its surfaces— 
either the water in which it is immersed, or the water taken 
into its alimentary canal. Its blood and all its juices being 
denser than water, the result is an osmotic absorption tend- 
ing ever to distend all its permeable parts—its tissues, 


THE INNER TISSUES OF ANIMALS. 329 


and its vessels when it has them. But these vessels and 
tissues are elastic; and if distended must everywhere com- 
press their contents—must tend, therefore, to squeeze out their 
contents where there is least resistance. Consequently, if at 
any place there is an abstraction of nutritive liquid, either 
for growth or function, more nutritive liquid will be forced 
towards that place. This cause of currents, which cannot 
fail to work throughout the distended tissues even of animals 
that are without blood-vessels, comes more actively into play 
where the body is everywhere traversed by these branching 
tubes with elastic walls. When we learn that the pressure 
of blood within the arteries and veins of a mammal varies 
from some 3 lbs. to + of a lb. per square inch, we see, on 
averaging this pressure, that the coats of the vascular system 
exert considerable force on the blood. This average pressure 
cannot be due to the heart’s action; since if, in the absence 
of the heart’s action, the whole mass of the blood in 
the vascular system were not above atmospheric pressure, 
the heart’s action could not produce a pressure above 
that of the atmosphere in one part of the vascular system 
without lowering the pressure below that of the atmo- 
sphere in another part of the vascular system. Hence 
it follows that irrespective of the heart’s action, the dis- 
tended walls of the vascular system must so compress 
the blood, as to cause a flow of it towards places 
where its escape is least resisted—towards places, that is, 
where it is most rapidly abstracted by function or growth. 
This is a cause of distribution which is at work before any 
central organ of circulation exists. Though in the rudimen- 
tary vascular systems of the simpler animals, the osmotic 
distension is probably nothing like so great, there must 
be some of it; and in the absence of a pumping organ, 
this force is probably an important aid to that move- 
ment of the blood which the functions set up. How 
the third cause—the changes of internal pressure which an 
animal’s movements produce—furthers circulation, will be 


830 PHYSIOLOGICAL DEVELOPMENT. 


sufficiently manifest. That parts which are bent or strained 
necessarily have their contained vessels squeezed, has been 
before shown (§ 281); and whether the bend or strain is 
caused, as in a plant, by an external force, or, as usually in an 
animal, by an internal force, there must be a thrusting of the 
liquids towards places of least resistance—that is, towards 
places of greatest consumption. This which in animals with- 
out hearts is a main agent of circulation, continues to further 
it very considerably even among the highest animals. There 
is experimental proof of the fact. The pressure in the jugu- 
lar vein of a horse, which is about 2 of a pound per 
square inch while the muscles are at rest, rises to 24 lbs. per 
square inch when the muscles are contracted to raise the 
head. Such, then, are the several forces we have to 
take into account in studying the genesis of the vascular 
system. Let us now pass to the facts to be interpreted. 

Even in such simple types as the Hydrozoa, cavities in the 
sarcode faintly indicate a structure that facilitates the transfer 
of nutritive matters. These vacuoles, possibly caused by the 
contraction of colloid substance in passing from the soluble 
to the insoluble state, become reservoirs filled with the 
plasma that slowly oozes through the sarcode; and every 
movement of the animal, accompanied as it must be by 
changed pressures and tensions on these reservoirs, tends 
here to fill them and there to squeeze out their contents in 
that or the other direction—possibly aiding to produce, by 
union of several vacuoles, those lacune or irregular canals 
which the sarcode in some cases presents. 

Irregular canals of this kind, not lined with any mem- 
branes but being simply cavities running through the flesh, 
mainly constitute the vascular system in Molluscoida and many 
Mollusca. In the simplest of these types the nutritive liquid, 
absorbed into the cavity of the peri-visceral sac, is thrust 
hither and thither through this sac with every change in the 
creature’s attitude, and simultaneously fills some of the 
sinuses which open out of this sac and run through the sub- 


THE INNER TISSUES OF ANIMAIS. ool 


stance of the body. ‘This distribution of the plasma, which 
muscular movement and osmotic distension here combine to 
aid, is, in somewhat more developed types, further aided by 
a rudimentary heart: in the peri-visceral sac is seated an 
open-mouthed tube, along which a wave of contraction pro- 
ceeds, first for a while in one direction and then again in the 
opposite direction. The higher orders of Mollusca have this 
simple contractile tube developed into a branched system of 
vessels or arteries, which run into the substance of the body 
and end in lacune or simple fissures. This ending in lacune 
takes place at various distances from the vascular centre. In 
some genera the arterial structure is carried to the periphery 
of the blood-system, while in others it stops short midway. 
Throughout most orders of the Mollusca the back current 
of blood continues to be carried by channels of the original 
kind: there are no true veins, but the blood having been 
delivered into the tissues, finds its way back to the peri-vis- 
ceral cavity through inosculating sinuses. Among the Ce- 
phalopods, however, the afferent blood-canals, as well as the 
efferent ones, acquire distinct walls; but even here the shut- 
ting off of the vascular system from the general cavity of the 
body is not complete; since there are still certain veins which 
empty themselves into the peri-visceral sac. Put- 
ting together these facts we may see pretty clearly the 
stages of vascular development. From the original reservoir 
of nutritive liquid between the alimentary canal and the wall 
of the body, a portion is partially shut off; and by the ver- 
micular contraction of the open tube thus formed, there is 
produced a more rapid transfer of the nutritive liquid from 
one part of the peri-visceral sac to another, than was origi- 
nally produced by the motions of the animal. Clearly, the 
extension of this contractile tube and the development from 
it of branches running hither and thither into the tissues, 
must, by defining the channels of the blood throughout a part 
of its course, render its distribution more regular and active. 
As fast as this centrifugal growth of definite channels advances, 


332 PHYSIOLOGICAL DEVELOPMENT. 


so fas. are the efferent currents of blood, prevented from 
escaping laterally, obliged to move from the centre towards 
the circumference ; and so fast also does the less-developed 
set of channels become, of necessity, occupied by afferent 
currents. When, by a parallel increase of definiteness, the 
lacunee and irregular sinuses through which the afferent cur- 
rents pass, become transformed into veins, the accompanying 
disappearance of all stagnant or slow-moving collections of 
blood, implies a furtner improvement in the circulation. 

By what agency is effected this differentiation of a definite 
vascular system from the indefinite peri-visceral sac? No 
sufficient reply is obvious. The genesis of the primordial 
heart is not comprehensible as a result of direct equilibration ; 
and we cannot readily see our way to it as a result of in- 
direct equilibration; for it is difficult to imagine what favour- 
able variation natural selection could have seized hold of to 
produce such a structure. A contractile tube that aided 
the distribution of nutritive liquid, being once established, 
survival of the fittest would suffice for its gradual extension 
and its successive modifications. But what were the early 
stages of the contractile tube, while it was yet not sufficiently 
formed to help circulation, and while it must nevertheless have 
had some advantage without which no selective process could 
goon? This part of the question we must leave as at present 
insoluble. To another part of the question, how- 
ever, an answer may be ventured. If we ask the origin of 
those ramifying channels which, first appearingas simple chan- 
nels, eventually become vessels having definite walls, a reply 
admitting of considerable justification, is, that the currents of 
nutritive liquid forced and drawn hither and thither through 
the tissues themselves initiate these channels. We know that 
streams running over and through solid and quasi-solid inor- 
ganic maiter, tend to excavate definite courses. We saw 
reason for concluding that the development of sap-channels 
in plants conforms to this general principle. May we not 
then suspect that the nutritive liquid contained in the tissue 


THE INNER TISSUES OF ANIMALS. 303 


of a simple animal, made to ooze now in this direction and | 
now in that by osmotic distension and by the changes of 
pressure which the animal’s movements cause, comes to have 
certain lines along which it is thrust backwards and forwards 
more than along other lines; and must by repeated passings 
make these more and more permeable, until they become 
lacune? Such actions will inevitably go on; and such actions 
appear competent to produce some, at least, of the observed 
effects. The leading facts which indicate that this is a part 
cause of vascular development, are these. 

Growths normally recurring in certain places at certain 
intervals, are accompanied by local formations of blood-vessels. 
The periodic maturation of ova among the Mammalia, supplies 
an instance. Through the stroma of an ovarium are dis- 
tributed innumerable minute vesicles, which, in their early 
stages, are microscopic. Of these, severally contained in their 
minute ovi-sacs, any one may develop: the determining 
cause being probably some slight excess of nutrition. When - 
the development is becoming rapid, the capillaries of the 
neighbouring stroma increase and form a plexus on the walls 
of the ovi-sac. Now since there is no typical distribution of 
the developing ova; and since the increase of an ovum to 
a certain size precedes the increase of vascularity round 
it; we can scarcely help concluding that the setting up 
of currents towards the point of growth determines the 
formation of the blood-vessels. It may be that having 
once commenced, this local vascular structure completes 
itself in a typical manner; but it seems clear that this 
greater development of blood-vessels around the growing 
ovum is initiated by the draught towards it. Ab- 
normal growths show still better this relation of cause and ef- 
fect. The false membranes sometimes found in the bronchial 
tubes in croup, may perhaps fairly be held abnormal in but a 
partial sense: it may be said that their vascular systems are 
formed after the type of the membranes to which they are 
akin. But this can scarcely be said of the morbid growths 


334 PHYSIOLOGICAL DEVELOPMENT. 


classed as malignant. ‘The blood-vessels in an encephaloid 
cancer, are led to enlarge and ramify, often to an immense 
extent, by the unfolding of the morbid mass to which they 
carry blood. Alien as is the structure as a whole to the type 
of the organism ; and alien in great measure as is its tissue 
to the tissue on which it is seated; it nevertheless happens 
that the growth of the alien tissue and accompanying ab- 
straction of materials from the blood-vessels, determine a 
corresponding growth of these blood-vessels. Unless, then, 
we say that there is a providentially-created type of vascular 
structure for each kind of morbid growth (and even this 
would not much help us, since the vascular structure has 
no constancy within the limits of each kind), we are com- 
pelled to admit that in some way or other the currents of 
blood are here directly instrumental in forming their own 
channels. One more piece of evidence, before cited 
as exemplifying adaptation (§ 67), may be called to mind. 
When any main channel for blood, leading to or from a 
certain part of the body, has been rendered impervious, 
others among the channels leading to or from this same part, 
enlarge to the extent requisite for fulfilling the extra fune- 
tion that falls upon them: the enlargement being caused, as 
we must infer, by the increase of the currents carried. 

Here then are facts warranting inductively the deduction 
above drawn. It is true that we are left in the dark respect- 
ing the complexities of the process. How the channels for 
blood come to have limiting membranes, and many of them 
muscular coats, the hypothesis does not help us to say. But 
the evidence assigned goes far to warrant the belief that vascu- 
lar development is initiated by direct equilibration ; though in- 
direct equilibration may have had the larger share in establish- 
ing the structures which distinguish finished vascular systems. 


§ 301. Of the inner tissues which remain let us next take 
bone. In what manner is differentiated this dense substance 
serving in most cases for internal support ? 


THE INNER TISSUES OF ANIMALS, 300 


Already when considering the vertebrate skeleton under 
its morphological aspect (§ 256) it was pointed out that the 
formation of dense tissues, internal as well as external, is, in 
some cases at least, brought about by the mechanical forces 
to be resisted. Through what process it is brought about we 
could not then stay to inquire: this question being not 
morphological but physiological. Answers to some kindred 
questions have since been attempted. Certain actions to 
which the internal dense tissues of plants may be ascribed, 
have been indicated ; and more recently, analogous actions 
have been assigned as causes of some external dense tissues 
of animals. We have now to ask whether actions of the 
same nature have produced these internal dense tissues of 
animals. 

The problem is an involved one. Bones have more than one 
stage: they are membranous or cartilaginous before they be- 
come osseous ; and their successive component substances so far 
differ that the effects of mechanical actions upon them differ. 
And having to deal with transitional states in which bone is 
formed of mixed tissues, having unlike physical properties 
and unlike minute structures, the effects of strains become 
too complicated to follow with precision. Anything in the 
way of interpretation must therefore be regarded as tentative. 
If analysis and comparison show that the phenomena are not 
inconsistent with the hypothesis of mechanical genesis, it is 
as much as can be expected. Let us first observe more nearly 
the mechanical conditions to which bones are subject. 

The endo-skeleton of a mammal with the muscles and liga- 
ments holding it together, may be rudely compared to a 
structure built up of struts and ties; of which, speaking 
generally, the struts bear the pressures and the ties bear the 
tensions. ‘The framework of an ordinary iron roof will give 
an idea of the functions of these two elements, and of the 
mechanical characters required by them. Such a framework 
consists partly of pieces that have each to bear a thrust in 
the direction of its length, and partly of pieces that have each 


336 PHYSIOLOGICAL DEVELOPMENT. 


to bear a pull in the direction of its length; and these struts 
and ties are differently formed to adapt them to these 
different strains. Further, it should be remarked that though 
the rigidity of the framework depends on the ties which are 
flexible, as much as on the struts which are stiff, yet the ties 
help to give the rigidity simply by so holding the struts in 
position that they cannot escape from the thrusts which fall 
on them. Now the like relation holds with a difference 
among the bones and muscles—the difference being, that here 
the ties admit of being lengthened or shortened and the struts 
of being moved about upon their joints. The mechanical re- 
lations are not altered by this however. The actions are of 
essentially the same kind in an animal that is standing, or 
keeping itself in a strained attitude, as in one that is changing 
its attitude—the same in so far that we have in each a set 
of flexible parts that are pulling and a set of rigid parts that 
are resisting. It needs but to remember the sudden collapse 
and fall that take place when the muscles are paralyzed, or 
to remember the inability of a bare skeleton to support itself, 
to see that the struts without the ties cannot suffice. And 
we have but to think of the formless mass into which a man 
would sink when deprived of his bones, to see that the ties 
without the struts cannot suffice. To trace the way in which 
a particular bone has its particular thrust thrown upon it, 
may not always be practicable. Though it is easy to perceive 
how a flexor or extensor of the arm causes by its tension a re- 
active pressure along the line of the humerus, and is enabled 
to produce its effect only by the rigidity of the humerus; yet 
it is not so easy to perceive how such bones as those of a 
horse’s haunch are similarly acted upon. Still, as the weight 
of the hind quarters has to be transferred from the pelvis to 
the feet, and must be so transferred through the bones, it is 
manifest that though these bones form a very crooked line, 
the weight must produce a pressure along the axis of each : the 
muscles and ligaments concerned serving here, as in other 
zases, so to hold the bones that they bear the pressure instead 


THE INNER TISSUES OF ANIMALS. 307 


of being displaced byit. Not forgetting that many processes 
of the bones have to bear tensions, we may then say that 
generally, though by no means universally, bones are in- 
ternal dense masses that have to bear pressures—pressures 
which in the cylindrical bones become longitudinal thrusts. 
Leaving out exceptional cases, let us consider bones as masses 
thus circumstanced. 

When giving reasons for the belief that the vertebrate 
skeleton is mechanically originated, one of the facts put in 
evidence was, that in the vertebrate series the transition from 
the cartilaginous to the osseous spine begins peripherally 
(§ 257): each vertebra being at first a ring of bone sur- 
rounding a mass of cartilage. And it was pointed out that 
this peripheral ossification is ossification at the region of 
greatest pressures. Now it is not vertebre only that follow 
this course of development. In a cylindrical bone, though 
it is differently circumstanced, the places of commencing ossi- 
fication are still the places on which the severest stress falls. 
Let us consider how such a bone that has to bear a longitu- 
dinal pressure is mechanica!y affected. If the end ofa 
walking-cane be thrust with force against the ground, the cane 
bends; and partially resuming its straightness when relieved, 
again bends, usually towards the same side, when the thrust 
is renewed. A bend so caused acts on the fibres of the cane 
in nearly the same way as does a bend caused by supporting 
the cane horizontally at its two ends and suspending a 
weight from its middle. In either case the fibres on the con- 
vex side are extended and the fibres on the concave side com- 
pressed. Kindred actions occur in a rod that is so thick as 
not to yield visibly under the force applied. In the absence 
of complete homogeneity of its substance, complete symmetry 
in its form, and an application of a force exactly along its 
axis, there must be some lateral deflection; and therefore 
some distribution of tensions and pressures of the kind indi- 
cated. And then, as the fact which here specially concerns us, 


we have to note that the strongest tensions and pressures are 
VOL. IL. 22 


COS PHYSIOLOGICAL DEVELOPMENT. 


borne by the outer layers of fibres. Now the shaft of a long 
bone, subject to mechanical actions of this kind, similarly has 
its outer layer most strained. In this layer, therefore, on the 
mechanical hypothesis, ossification should commence, and here 


it does commence—commences, too, midway between the ends 
where the bends produce on the superficial parts their most 
intense effects. But we have not in this place simply 
to observe that ossification commences at the places of greatest 
stress, but to ask what causes it to do this. Can we trace the 
physical actions which set up this deposit of dense tissue? It 
is, I think, possible to indicate a “‘ true cause” that is at work ; 
though whether it is a sufficient cause may be questioned. 
We concluded that in certain other cases, the formation of 
dense tissue indirectly results from the alternate squeezing 
and relaxation of the vessels running through the part; and 
the inquiry now to be made is, whether, in developing bone, 
the same actions go on in such ways as to produce the ob- 
served effects. At the outset we are met by what seems a 
fatal difficulty—cartilage is a non-vascular tissue: this sub- 
stance of which unossified bones consist 1s not permeated by 
minute canals carrying nutritive liquid, and cannot, there- 
fore, be a seat of actions such as those assigned. This ap- 
parent difficulty, however, furnishes a confirmation. For 
cartilage that is wholly without blood-vessels does not ossify : 
ossification takes place only at those parts of it into 
which the capillaries penetrate. Hence, we get additional 
reason for suspecting that bone-formation is due to the alleged 
cause; since it occurs where mechanical strains can produce 
the actions described, but does not occur where mechanica 

strains cannot produce them. Let us consider more closel 

what the factors are, and how they will codperate under 
the particular conditions. It seems possible that 
these canals that exist in the superficial layer of a cartilagin- 
ous bone before it begins to ossify, are themselves produced 
by the mechanical actions. For every time a mass of carti- 
lage is strained, and its superficial layers more especially 


THE INNER TISSUES OF ANIMALS. 339 


subject to tensions and pressures, the nutritive liquid diffused 
through the substance of the cartilage, compressed as it must 
be, will tend to ooze from the surface of the cartilage, and to 
return again when the stress is taken off. Such alternate 
movements of the nutritive liquid, perpetually repeated, will 
be apt to form channels. These, at first quite superficial and 
inappreciable, will become more appreciable; since, when 
they are once commenced, any further additions of substance 
to the surface will be prevented from closing their openings 
by the alternate rushes of liquid; and so a vascular layer 
of appreciable thickness may gradually be formed. But 
without doing more than hint this, it will suffice for the 
argument if we commence with the external vascular layer 
as already existing, and consider what will take place in 
it. Cartilage is elastic—is somewhat extensible, and 
spreads out laterally under pressure, but resumes its form 
when relieved. How, then, will the capillaries traversing 
such a substance be affected at the places where it is strained 
by a bend? Those on the convex side will be laterally 
squeezed, in the same way that we saw the sap-vessels on the 
convex side of a bent branch are squeezed ; and as exudation 
of the sap into the adjacent prosenchyma will be caused in 
the one case, so, in the other, there will be caused exudation 
of serum into the adjacent cartilage: extra nutrition and 
increase of strength resulting in both cases. The parallel 
ceases here, however. In the shoot of a plant, bent in 
various directions by the wind, the side which was lately 
compressed, is now extended; and hence that squeezing 
of the sap-vessels which results from extension, suffices to 
feed and harden the tissue on all sides of the shoot. But it 
is not so with a bone. Having yielded on one side under 
longitudinal pressure, and resumed as nearly as may be its 
previous shape when the pressure is taken off, the bone yields 
again towards the same side when again longitudinally 
pressed. Hence the substance of its concave side, never 


rendered convex by a bend in the opposite direction, would 
22% 


340 PHYSIOLOGICAL DEVELOPMENT, 


not receive any extra nutrition did no other action come into 
play. Butif we consider how intermittent pressures must 
act on cartilage, we shall see that there will result extra 
nutrition of the concave side also. Squeeze between two 
pieces of glass a thin bit of caoutchouc that has a hole 
through it. While the caoutchouc spreads out away from 
the centre, it also spreads inwards, so as partially to close the 
hole. Everywhere its molecules move away in directions of 
least resistance; and for those near the hole, the direction of 
least resistance is towards the hole. Let this hole stand for 
the transverse section of one of the capillaries passing - 
through cartilage, and it wili be manifest that on the side of 
the unossified bone made concave in the way described, the 
compressed cartilage will squeeze the capillaries traversing 
it; and in the absence of perfect homogeneity in the 
cartilage, the squeeze will cause extra exudation from the 
capillaries into the cartilage. Thus every additional strain 
will give to the cartilage it falls upon, an additional supply 
of the materials for growth. So that presently the side 
which, by yielding more than any other, proves itself to be 
the weakest, will cease to be the weakest. What further will 
happen? Some other side will yield a little—the bends will 
take place in some other plane; and the portions of cartilage 
on which repeated tensions and pressures now fall will be 
strengthened. Thus the rate of nutrition, greatest at the 
place where the bending is greatest, and changing as the 
incidence of forces changes, will bring about at every point a 
balance between the resistances and the strains. Thus, too, 
there will be determined that peripheral induration which we 
see in bones so circumstanced. As in a shoot we saw that the 
woody deposit takes place towards the outside of the cylinder, 
where, according to the hypothesis, it ought to take place; 
so, here, we see that the excess of exudation and hardening, 
occurring where the strains are most intense, will form a 
cylinder having a dense outside and a porous or hollow 
inside. These processes will be essentially the same 


THE INNER TISSUES OF ANIMALS. 341 


in bones subject to more complex mechanical actions; such 
as sundry of the flat bones and others that serve as internal 
fuicra. Be the strains transverse or longitudinal, be they 
torsion strains or mixed strains, the outer parts of the bone 
will be more affected by them than its inner parts. They 
will therefore tend everywhere to produce resisting masses 
having outer parts more dense than their inner parts. And 
by causing most growth where they are most intense, will 
call out reactive forces adequate to balance them—forms and 
thicknesses of bone offering resistances equal to the strains, 
however numerous and varied. There are doubt- 
less obstacles in the way of this interpretation. It may be 
said that the forces acting on the outer layers in the manner 
described, would compress the capillaries too little to produce 
the alleged effects ; and if evenly distributed along the whole 
lengths of the layers, they would probably be so. But it 
needs only to bend a flexible mass and observe the tendency 
to form creases on the concave surface, to feel assured that 
along the surface of an ossifying bone, the yielding of the 
tissue when bent will not be uniform. In the absence of 
complete homogeneity, the interstitial yielding will take 
place at some points more than others, and at one point above 
all others. At these weakest points, and especially at one, 
the action on the capillaries will be concentrated. When, 
at the weakest point—the centre of commencing ossification 
—an extra amount of deposit has been caused, it will cease 
to be the weakest ; and adjacent points, now the weakest, will 
become the places of yielding and induration. And in pro- 
portion as the layer becomes filled with unyielding matter, 
the remaining compressible parts of it, and their contained 
capillaries, will be more severely compressed. It may be 
further objected that the hypothesis is incompatible with the 
persistence of cartilage for so long a time between the 
epiphyses of bones and the bony masses which they ter- 
minate. But there is the reply that the places occupied by 
this cartilage, being places at which the bone lengthens, the 


342 PHYSIOLOGICAL DEVELOPMENT. 


non-ossification is in part apparent only—it is rather that 
new cartilage is formed as fast as the pre-existing cartilage 
ossifies ; and there is the further reply that the slowness of 
the ultimate ossification of this part, is due to its non- 
vascularity, and to mechanical conditions that are unfayour- 
able to its acquirement of vascularity. Once more, the de- 
murrer that in the epiphyses ossification does not begin at 
the surface but within the mass of the cartilage, is met by an 
explanation parallel to that before given (§ 293, note) of the 
deep-seated induration produced by an external pressure 
which, during long intervals, does not intermit completely ; 
as in a bunion, a node on the instep, and what is called 
‘“‘housemaid’s knee.” 

Of course it is not meant that this osseous development by 
direct equilibration, takes place in the individual. Though 
it is a corollary from the argument that in each individual 
the process must be furthered and modified by the particular 
actions to which the particular bones are exposed; yet the 
leading traits of structure assumed by the bones are assumed 
in conformity with the inherited type. This, however, is no 
difficulty. The type itself1s to be regarded as the accumulated 


result of such modifications, transmitted and increased from 


generation to generation. The actions above described as 
taking place in the bone of an individual, must be understood 
as producing their total effect little by little in the corre- 
sponding bones of a long series of individuals. Even if but 
a small modification can be so wrought in the individual, yet 
if such modification, or a part of it, is inheritable, we may 
readily understand how, in the course of geologic epochs, the 
observed structures may arise by the assigned way. 

Here may fitly come in a strong confirmation. If we find 
cases Where individual bones, subject in exceptional degrees 
to the actions described, present in exceptional amounts the 
modifications attributed to them, we are greatly helped in 
understanding how there may be produced in the race that 
aggregate of modifications which the hypothesis implies. 


THE INNER TISSUES OF ANIMALS. 3438 


Such cases occur in ricketty children. I am indebted to Mr. 
Busk for pointing out these abnormal formations of dense 
tissue, that are not apparently explicable as results of | 
mechanical actions and re-actions. It was only on tracing 
out the processes here at work, that there suggested itself the 
specific interpretation of the normal process, as above set 
forth. When, from constitutional defect, bones do 
not ossify with due rapidity, and are meanwhile subject to 
the ordinary strains, they become distorted. Remembering 
how a mass which has been made to yield in any direction 
by a force it cannot withstand, is some little time before it 
recovers completely its previous form, and usually, indeed, 
undergoes what is called a “ permanent set ;”’ it is inferable 
that when a bone is repeatedly bent at the same time that 
the liquid contained in its capillaries is poor in the materials 
for forming dense tissue, there will not take place a propor- 
tionate strengthening of the parts most strained ; and these 
parts will give way. This happens in rickets. But this 
having happened, there goes on what, in teleological language, 
we call a remedial process. Supposing the bone to be one 
commonly affected—a femur; and supposing a permanent 
bend to have been caused in it by the weight of the body ; 
the subsequent result is an unusual deposition of cartilaginous 
and osseous matter on the concave side of the bone. If the 
bone is represented by a strung bow, then the deposit occurs 
at the part represented by the space between the bow and 
the string. And thus occurring where its resistance is most 
effective, it increases until the approximately-straight piece 
of bone formed within the arc, has become strong enough to 
bear the pressure without appreciably yielding. Now 
this direct adaptation, seeming so like a special provision, 
and furnishing so remarkable an instance of what, in medical 
but unscientific language, is called the vis medicatrix nature, 
is simply a result of the above-described mechanical actions 
and re-actions, going on under the exceptional conditions. 
Each time such a bent bone is subject to a force which again 


O44 PHYSIOLOGICAL DEVELOPMENT. 


bends it, the severest compression falls on the substance of 
its concave side. Each time, then, the capillaries running 
through this part of its substance are violently squeezed— 
far more squeezed than they or any other of the capillaries. 
would have been, had the bone remained straight. Hence, 
on every repetition of the strain, these capillaries near the 
concave surface have their contents forced out in more 
than normal abundance. The materials for the formation of 
tissue are supplied in quantity greater than can be assimi- 
lated by the tissue already formed ; and from the excess of 
exuded plasma, new tissue arises. A layer of organizable 
material accumulates between the concave surface and the 
periosteum; in this, according to the ordinary course of 
tissue-growth, new capillaries appear; and the added layer 
presently assumes the histological character of the layer from 
which it has grown. What next happens? This added 
layer, further from the neutral axis than that which has 
thrown it out, is now the most severely compressed, and its. 
capillaries are the most severely squeezed. The place of 
greatest exudation and most rapid deposit of matter, is there- 
fore transferred to this new layer; and at the same time that 
active nutrition increases its density, the excess of organizable 
- material forms another layer external to it: the successive 
layers so added, encroaching on the space between the concave 
surface of the bone and the chord of its are. ‘What 
limits the encroachment on this space ?—what stops the pro- 
cess of filling it up? ‘The answer to this question will be 
manifest on observing that there comes into play a cause 
which gradually diminishes the forces falling on each new 
layer. For the transverse sectional area is step by step 
increased ; and an increase of the area over which the weight 
borne is distributed, implies a relatively smaller pressure 
upon each part of it. Further, as the transverse dimensions 
of the bone increase, the materials composing its convex and 
concave layers, becoming further from the neutral axis, 
become better placed for resisting the strains to be borne. 


THE INNER TISSUES OF ANIMALS. 345 


So that both by the increased quantity of dense matter and 
by its mechanically more-advantageous position, the bendings 
of the bone are progressively decreased. But as they are 
decreased, each new layer formed on the concave surface, has 
its substance and *is capillaries less compressed; and the 
resulting growta and induration are rendered less rapid. 
Evidently, then, the additions, slowly diminishing, will 
eventually cease; and this will happen when the bone no 
longer bends. That is to say, the thickening of the bone will 
reach its limit when there is equilibrium between the inci- 
dent forces and the forces which resist them. Here, indeed, 
we may trace with great clearness the process of direct 
equilibration—may see how an unusual force, falling on the 
moving equilibrium of an. organism and not overthrowing it, 
goes on working modifications until the re-action balances 
the action. 

That, however, which now chiefly concerns us, is to note 
how this marked adaptation supports the general argument. 
Unquestionably bone is in this case formed under the influ- 
ence of mechanical stress, and formed just where it most 
effectually meets the stress. This result, not otherwise 
explained, is explained by the hypothesis above set forth. 
And when we sce that this special deposit of bone is ac- 
counted for by actions like those to which bone-formation in 
general is ascribed, the probability that these are the actions 
at work becomes very great. 

Of course it is not alleged that osseous structures arise in 
this way alone. The bones of the skull and various dermal 
bones cannot be thus interpreted. Here the natural selec- 
tion of favourable variations appears the only assignable 
cause—the equilibration is indirect. We know that ossific 
deposits now and then occur in tissues where they are not 
usually found; and such deposits, originally abnormal, if 
they occurred in places where advantages arose from them, 
might readily be established and increased by survival of the 
fittest. Especially might we expect this to happen when a 


346 PHYSIOLOGICAL DEVELOPMENT. 


constitutional tendency to form bone had been established by 
actions of the kind deseribed; for it is a familiar fact that 
differentiated types of tissue, having once become elements 
of an organism, are apt occasionally to arise in unusual 
places, and there to repeat all their peculiar histological cha- 
racters. And this may possibly be the reason why the bones 
of the skull, though not exposed to forces such as those which 
produce, in other bones, dense outer layers including less 
dense interiors, nevertheless repeat this general trait of bony 
structure. While, however, it is beyond doubt that some 
bones are not due to the direct influence of mechanical stress, 
we may, I think, conclude that mechanical stress initiates 
bone-formation. 


§ 302. What is the origin of nerve? In what way do its 
properties stand related to the properties of that protoplasm 
whence the tissues in general arise? and in what way is it 
differentiated from protoplasm simultaneously with the other 
tissues? These are profoundly interesting questions; but 
questions to which positive answers cannot be expected. 
All that can be done is to indicate answers which seem 
feasible. 

That the property specially displayed by nerve, is a pro- 
perty which protoplasm possesses in a lower degree, is mani- 
fest. The sarcode of a Rhizopod and the substance of an 
unimpregnated ovum, exhibit movements that imply a propa- 
gation of stimulus from one part of the mass to another ; and 
through the nerveless body of a polype, we see slowly 
travelling and spreading a contraction set up by touching a 
tentacle—a contraction which implies the passage from part to 
part of some stimulus causing the contraction. We 
have not far to seek for a probable origin of this phenomenon. 
There is good reason for ascribing it to the extreme insta- 
bility of the organic colloids of which protoplasm consists. 
These, in common with colloids in general, assume different 
isomeric forms with great facility; and they display not 


THE TNNER TISSUES OF ANIMALS, 347 


simply isomerism but polymerism. Further, this readiness to 
undergo molecular re-arrangement, habitually shows itself in 
colloids by the rapid propagation of the re-arrangement 
from part to part. As Prof. Graham has shown, matter in 
this state often “ pectizes’”’ almost instantaneously—a touch 
will transform an entire mass. That is to say, the change of 
molecular state once set up at one end, spreads to the other 
end—there is a progress of a stimulus to change; and this is 
what we see in a nerve. So much being understood, let us 
re-state the case more completely. 

Molecular change, implying as it does motion of molecules, 
communicates motion to adjacent molecules; be they of the 
same kind or of a different kind. If the adjacent molecules, 
either of the same kind or of a different kind, be stable in 
composition, a temporary increase of oscillation in them as 
wholes, or in their parts, may be the only result ; but if they 
are unstable there are apt to arise changes of arrangement 
among them, or among their parts, of more or less permanent 
kinds. Especially is this so with the complex molecules 
which form colloidal matter, and with the organic colloids 
above all. Hence it is to be inferred that a molecular dis- 
turbance in any part of a living animal, set up by either an 
external or internal agency, will almost certainly disturb and 
change some of the surrounding colloids not originally im- 
plicated—will diffuse a wave of change towards other parts 
of the organism: a wave which will, in the absence of per- 
fect homogeneity, travel further in some directions than in 
others. Let us ask next what will determine the 
differences of distance travelled in different directions. Ob- 
viously any molecular agitation spreading from a centre, will 
go furthest along routes that offer least resistance. What routes 
will these be? Those along which there lie most molecules 
that are easily changed by the diffused molecular motion, and 
which yet do not take up much molecular motion in assuming 
their new states. Molecules which are tolerably stable will 
not readily propagate the agitation ; for they will absorb it 


348 PHYSIOLOGICAL DEVELOPMENT. 


in the increase of their own oscillations, instead of passing i6 
on. Molecules which are unstable but which, in assuming 
isomeric forms, absorb motion, will not readily propagate it ; 
since it will disappear in working the changes in them. But 
unstable molecules which, in being isomerically transformed, 
do not absorb motion, and still more those which, in being 
so transformed, give out motion, will readily propagate any 
molecular agitation ; since they will pass on the impulse either 
undiminished, or increased, to adjacent molecules. if, 
then we assume, as we are not only warranted in doing but 
are obliged to do, that protoplasm contains two or more 
colloids, either mingled or feebly combined (since it cannot 
consist of simple albumen or fibrin or casein, or any allied 
proximate principle) ; it may be concluded that any mole- 
cular agitation set up by what we call a stimulus, will diffuse 
itself further along some lines than along others, if the com- 
ponents of the protoplasm are not quite homogeneously dis- 
persed, and if some of them are isomerically transformed 
more easily, or with less expenditure of motion, than 
others; and it will especially travel along spaces occupied 
chiefly by those molecules which give out molecular mo- 
tion during their metamorphoses, if there should be any 
such. But now let us ask what structural effects 
will be wrought along a tract traversed by this wave of | 
molecular disturbance. As is shown by those transforma- 
tions that so rapidly propagate themselves through colloids, 
molecules that have undergone a certain change of form, 
are apt to communicate a like change of form to ad- 
jacent molecules of the same kind—the impact of each 
overthrow is passed on and produces another overthrow. 
Probably the proneness towards isochronism of molecular 
movements necessitates this. If any molecule has had 
its components re-arranged, and their oscillations conse- 
quently altered, there result movements not concordant with 
the movements in adjacent untransformed molecules, but 
which, impressing themselves on the parts of such untrans- 


THE INNER TISSUES OF ANIMALS. 049 


formed molecules, tend to generate in them concordant move- 
ments—tend, that is, to produce the re-arrangements involved 
by these concordant movements. Is this action limited to 
strictly isomeric substances P or may it extend to substances 
that are closely allied? If along with the molecules of a 
compound colloid there are mingled those of some kindred 
colloid; or if with the molecules of this compound colloid 
there are mingled the components out of which other such 
molecules may be formed; then there arises the question— 
does the same influence which tends to propagate the iso- 
meric transformations, tend also to form new molecules of 
the same kind out of the adjacent components? There is 
reason to suspect that it does. Already when treating of the 
nutrition of parts (§ 64), it was pointed out that we are obliged 
to recognize a power possessed by each tissue to build up, out 
of the materials brought to it, molecules of the same type as 
those of which it is formed. This building up of like mole- 
cules seems explicable as caused by the tendency of the 
new components which the blood supplies, to acquire move- 
ments isochronous with those of the like components in the 
tissue ; which they can do only by uniting into like com- 
pound molecules. Necessarily they must gravitate towards a 
state of equilibrium; such state of equilibrium—moving 
equilibrium of course—must be one in which they oscillate 
in the same times with neighbouring molecules; and so 
to oscillate they must fall into groups identical with the 
groups around them. If this be a general principle of 
tissue-growth and repair, we may conclude that it will apply 
in the case before us. A wave of molecular disturbance 
passing along a tract of mingled colloids closely allied in com- 
position, and isomerically transforming the molecules of one 
of them, will be apt at the same time to form some new mole- 
cules of the same type, at any place where there exist the 
proximate components, either uncombined or feebly combined 
in some not very different way. And this will be most likely 
to occur where the molecules of the colloid that are under- 


300 FHYSIOLOGICAL DEVELOPMENT. 


going the isomeric change, predominate, but have scattered 
through them the other molecules out of which they may be 
formed, either by composition or modification. That is to 
say, a wave of molecular disturbance diffused from a centre, 
and travelling furthest along a line where lie most molecules 
that can be isomerically transformed with facility, will be 
likely at the same time to further differentiate this line, and 
make it more characterized than before by the easy-trans- 
formability of its molecules. One additional step, 
and the interpretation is reached. Analogy shows it to be 
not improbable that these »rganic colloids, isomerically trans- 
formed by slight molecular impact or increase of molecular 
motion, will some of them resume their previous molecular 
structures after the disturbance has passed. We know that 
what are stable molecular arrangements under one degree of 
molecular agitation, are not stable under another degree ; and 
there is evidence that re-arrangements of an inconspicuous 
kind are occasionally brought about by very slight changes 
of molecular agitation. Water supplies a case. Prof. 
Graham infers that water undergoes a molecular re-arrange- 
ment at about 32°—that ice has a colloid form as well as a 
crystalloid form, dependent on temperature. Send through 
it an extra wave of the molecular agitation we call heat, and 
its molecules aggregate in one way. Let the wave die.away, 
and its molecules resume their previous mode of aggregation. 
And obviously such transformations may be repeated back- 
wards and forwards within narrow limits of temperature. 
Now among the extremely unstable organic colloids, such a 
phenomenon is far more likely to happen. Suppose, then, that 
the nerve-colloid is one of which the molecules are changed in 
form by a passing wave of extra agitation, but resume their 

revious form when the wave has passed: the previous form 
being the most stable under the conditions which then recur. 
What follows? It follows that these molecules will be ready 
again to undergo isomeric transformation when there again 
occurs the stimulus; will, as before, propagate the transforma- 


ee 


THE INNER TISSUES OF ANIMALS. Jol 


tion most along the tract where they are most abundant ; will, 
as before, simultaneously tend to form new molecules of their 
own type; will, as before, make the line along which they lie 
one of easier transfer for the molecular agitation. Every 
repetition will help to increase, to integrate, to define more 
completely, the course of the escaping molecular motion—- 
extending its remoter part while it makes its nearer part 
more permeable—will help, that is, to form a line of discharge, 
a line for conducting impressions, a nerve. 

Such seems to me a not unfair series of deductions from 
the known habitudes of colloids in general and the organic 
colloids in particular. And I think that the implied nature 
and properties of nerve, correspond better with the observed 
phenomena than do the nature and properties implied by 
other hypotheses, Of course the speculation as it here stands 
is but tentative, and leaves much unexplained. It gives no 
obvious reply to the questions—what causes the formation of 
nerves along some lines rather than others ? what determines 
their appropriate connexions P—-questions, however, to which, 
when we come to deal with physiological integration, we may 
find not unsatisfactory answers. Moreover it says nothing 
about the genesis of ganglia. A ganglion, it is clear, must 
consist of a colloidal matter equally unstable, or stiil more 
unstable, which, when disturbed, falls into some different 
molecular arrangement, perhaps chemically simpler, and gives 
out in so doing a large amount of molecular motion—serves 
as a reservoir of molecular motion which may be suddenly 
discharged along an efferent nerve or nerves, when excite- 
ment of an afferent nerve has disengaged it. How such 
a structure as this results, the hypothesis does not show. 
But admitting these shortcomings it may still be held that 
we are, in the way pointed out, enabled to form an idea of 
the actions by which nervous tissue is differentiated. 


§ 3035. A speculation akin to, and continuous with, the last, 
is suggested by an inquiry into the origin of muscular tissue. 


O02 PHYSIOLOGICAL DEVELOPMENT. 


Contractility as well as irritability is a property of protoplasm 
or sarcode; and, as before suggested (§ 22), is not improbably 
due to isomeric change in one of its component colloids. It 
is a feasible supposition that of the several isomeric changes 
simultaneously set up among these component colloids, some 
may be accompanied by decided change of bulk and some not. 
Clearly the isomeric change undergone by the colloid which 
we suppose to form nerve, must be one not accompanied by 
appreciable change of bulk; since change of bulk implies 
‘internal work,” as physicists term it, and therefore ex- 
penditure of force. Conversely, the colloid out of which 
muscle originates, may be one that readily passes into an iso- 
meric state in which it occupies less space: the molecular — 
disturbance causing this contraction being communicated to 
it from adjacent portions of nerve-substance that are mole- 
cularly disturbed; or being otherwise communicated to it by 
direct mechanical or chemical stimuli; as happens where 
nerves do not exist, or where their influence has been cut 
off. This interpretation seems, indeed, to be directly at 
variance with the fact that muscle does not diminish in bulk 
during contraction but merely changes its shape. That which 
we see take place with the muscle as a whole, is said also to 
take place with each fibre—while it shortens it also broadens. 
There is, however, a possime solution of this difficulty. A 
contracting colloid yields up its water; and the contracted 
colloid plus the free water, may have the same bulk as before 
though the colloid has less. If it be replied that in this 
case the water should become visible between the substance 
of the fibre and its sarcolemma or sheath, it may be rejoined 
that this is not necessary—it may be deposited interstitially. 
Possibly the striated structure is one vhat facilitates its 
exudation and subsequent re-absorption ; and to this may be 
due the superiority of striated muscle in rapidity of contrac- 
tion. Granting the speculative character of this 
interpretation, let us see how far it agrees with the facts. If 
the actions are as here supposed, the contracted or more inte- 


fy € € 
Ve THE INNER LISSUES OF ANIMALS. 353 


grated state of the muscular colloid will be that which it 
tends continually to assume—that into which it has an in- 
creasing aptitude to pass when artificial paralysis has been 
produced, as shown by Dr. Norris—that into which it lapses 
completely in rigor mortis. The sensible motion generated 
by the contraction can arise only from the transformation 
of insensible motion. This insensible motion suddenly 
yielded up by a contracting mass, implies the fall of its com- 
ponent molecules into more stable arrangements. And there 
can be no such fall unless the previous arrangement is un- 
stable. From this point of view, too, it is pos- 
sible to see how the hydro-carbons and oxy-hydro-carbons 
consumed in muscular action, may produce their effects. It 
was said, when exposing The Data of Biology, that non-nitro- 
genous substance might evolve heat only when transformed 
in the circulating fluids, “ but partly heat, and partly another 
force, when transformed in some active tissue that has ab- 
sorbed it: just as coal, though producing little else but heat 
as ordinarily burnt, has its heat partially transformed into 
mechanical motion if burnt in a steam-engine furnace ” 
(§ 18); and recent inquiries make it clear that some such 
relation exists.* Here a feasible modus operandi becomes 
manifest. For these non-nitrogenous elements of food when 
consumed in the tissues, give out large amounts of molecular 
motion. They do this in presence of the muscular colloids 
that have lost molecular motion during their fall in the stable 
or contracted state. And from the molecular motion they 
give out, may be restored the molecular motion lost by 
the contracted colloids: these contracted colloids may 
so have their molecules raised to that unstable state from 
which, again falling, they can again generate mechanical 
motion. 


* See account of experiments made by Profs. Fick and Wislicenus, trans- 
lated by Prof. Wanklyn in the Phil. Mag. for May or June, 1866. See 
also an article by Prof. Frankland in the September number of the same 
journal, 


NOE.) Fis 23 


354 PHYSIOLOGICAL DEVELOPMENT. 


This conception of the nature and mode of action of muscle, 
while it is suggested by known properties of colloidal matter 
and conforms to the recent conclusions of organic chemistry 
and molecular physics, establishes a comprehensible relation 
between the vital actions of the lower and the higher animals. 
Tf we contemplate the movements of cilia, of a Rhizopod’s 
pseudo-podia, of a Polype’s body, or of the long pendant ten- 
tucles of a Medusa, we shall see great congruity between 
them and this hypothesis. Bearing in mind that the con- 
tractile substance of developed muscle is affected not by 
nervous influence only, but, where nervous influence is 
destroyed, is made to contract by mechanical disturbance and. 
chemical action, we may infer that it does not differ intrin- 
sically from the primordial contractile substance, which, in 
the lowest animals, changes its bulk under other stimuli than 
the nervous. We shall see significance in the fact ascer- 
tained by Dr. Ransom, that various agents which excite 
and arrest nervo-muscular movements in developed animals, 
excite and arrest the protoplasmic movements in ova. We 
shall understand how tissues not yet differentiated into muscle 
and nerve, have this joint irritability and contractility; how 
muscle and nerve may arise by the segregation of their 
mingled colloids, the one of which, not appreciably altering 
its bulk during isomeric change, readily propagates molecular 
disturbance, while the other, contracting when isomerically 
changed, less readily passes on the molecular disturbance; 
and how by this differentiation and integration of the con- 
ducting and the contracting colloids, the one ramifying 
through the other, it becomes possible for a whole mass to 
contract suddenly, instead of contracting gradually, as it does 
when undifferentiated. 

The question remaining to be asked is—What causes the 
specialization of contractile substance P—What causes the 
growth of colloid masses which monopolize this contractility, 
and leave kindred colloids to monopolize other properties ? 
Has natural selection -gradually localized and increased 


THE INNER TISSUES OF ANIMALS. 355 


the primordial muscular substance ? or has the frequent recur- 
rence of irritations and consequent contractions at particular 
parts done it? We have, I think, reason to conclude that 
direct equilibration rather than indirect equilibration has been 
chiefly operative. The reasoning that was used in the case 
of nerve applies equally in the case of muscle. A portion of 
undifferentiated tissue containing a predominance of the colloid 
that contracts in changing, will, during each change, tend 
to form new molecules of its own type from the other colloids 
diffused through it: the tendency of these entangled colloids 
to fall into unity with those around them, will be aided by 
every shock of isomeric transformation. Hence, repeated 
contractions will further the growth of the contracting mass, 
and advance its differentiation and integration. If, 
too, we remember that the muscular colloid is made to 
contract by mechanical disturbance, and that among me- 
chanical disturbances one which will most readily affect it 
simultaneously throughout its mass is caused by stretching, 
we shall be considerably helped towards understanding how 
the contractile tissues are developed. If extension of a mus- 
cular colloid previously at rest, produces in it that molecular 
disturbance that leads to isomeric change and decrease of 
bulk, then there is no difficulty in explaining the movements 
of cilia. The formation of a contractile layer in the vascular 
system becomes comprehensible: each dilatation of a blood- 
vessel caused by a gush of blood, will be followed by a con- 
striction ; the heart will pulsate violently in proportion as 
it is violently distended; arteries will develop in power as 
the stress upon them becomes greater. And we shall simi- 
larly have an explanation of the increased muscularity of 
the alimentary canal that is brought about by increased 
distension of it. 

That the production of contractile tissue in certain localities, 
is due to the more frequent excitement in those localities 
of the contractility possessed by undifferentiated tissue in 
general, is a view harmonizing with facts which the diffe- 

23 * | 


356 PHYSIOLOGICAL DEVELOPMENT. 


rentiated contractile tissues exhibit. These are the rela- 
tions between muscular exercise, muscular power, and mus- 
cular structure; and it is the more needful for us here to 
notice them because of certain anomalies they present, 
which, at first sight, seem inconsistent with the belief that 
the functionally-determined modifications of muscle are in- 
heritable. 

Muscles disagree greatly in their tints—all gradations 
between white and deep red being observable. Contrasts 
are visible between the muscles of different animals, be- 
tween the muscles of the same animal at different ages, and 
between different muscles of the same animal at the same 
age. We will glance at the facts under these heads: noting 
under each of them the connexion which here chiefly con- 
cerns us—that between the activity of muscle and its depth 
of colour. The cold-blooded Vertebrata are, taken 
as a group, distinguished from the warm-blooded by the 
whiteness of their flesh ; and they are also distinguished by 
their comparative inertness. Though a fish or a reptile can 
exert considerable force for a short time, it is not capable of 
prolonged exertion. Birds and mammals show greater en- 
durance along with darker-coloured muscles. If among birds 
themselves or mammals themselves we make comparisons, we 
meet with kindred contrasts—especially between wild and 
domestic creatures of allied kinds. Barn-door fowls are 
lighter-fleshed than most untamed gallinaceous birds; and 
among these last the pheasant, moving about but little, is 
lighter-fleshed than the partridge and the grouse which are 
more nomadic. The muscles of the sheep are not on the 
average so’ dark as those of the deer; and it is said that the 
flesh of the wild-boar is darker than that of the pig. 
Perhaps, however, the contrast between the hare and the 
rabbit affords, among familiar animals, the best example of 
the alleged rejation: the dark-fleshed hare having no retreat 
and making wide excursions, while the white-fleshed rabbit, 
passing a great part of its time in its burrow, rarely wanders 


THE INNER TISSUES OF ANIMALS. sta" 


far from home. The parallel contrast between 
young and old animals has a parallel meaning. Veal is 
much whiter than beef, and lamb is of lighter colour than 
mutton. Though at first sight these facts may not seem 
to furnish confirmatory evidence, since lambs in their play 
appear to expend more muscular force than their sedate 
dams; yet the meaning of the contrast is: really as alleged. 
For in consequence of the law that the strains which animals 
have to overcome, increase as the cubes of the dimensions, 
while their powers of overcoming them increase only as the 
squares (§ 46), the movements of an adult animal cost very 
much more in muscular effort than do those of a young 
animal: the result being that the sheep and the cow exercise 
their muscles more vigorously in their quiet movements, than 
the lamb and the calf in their lively movements. It may be 
added as significant, that the domestic animal in which no 
very marked darkening of the flesh takes place along with 
increasing age, namely the pig, is one which, ordinarily kept 
in a sty, leads so quiescent a life that the assigned cause of 
darkening does not come into action. But perhaps 
the most conclusive evidences are the contrasts that exist 
between the active and inactive muscles of the same animal. 
Between the leg-muscles of fowls and their pectoral muscles, 
the difference of colour is familiar; and we know that fowls 
exercise their leg-muscles much more than the muscles which 
move their wings. Similarly in the turkey, in the guinea 
fowl,.in the pheasant. And then, adding much to the force of 
this evidence, we see that in partridges and grouse, which 
belong to the same order as our domestic fowls, but use their 
wings as habitually as their legs, little or no difference is 
visible between the colours of these two groups of muscles. 
Special contrasts like these do not, however, exhaust the 
proofs ; for there is a still more significent general contrast. 
The muscle of the heart, which is the most active of all 
muscles, is the darkest of all muscles. 

_ The connection of phenomena thus shown in so many ways, 


358 © PHYSIOLOGICAL DEVELOPMENT. 


implies that the bulk of a muscle is by no means the sole 
measure of the quantity of force it can evolve. It would seem 
that, other things equal, the depth of colour varies with the — 
constancy of action; while, other things equal, the bulk varies 
with the amount of force that has to be put forth upon oc- 
casion. ‘These of course are approximate relations. More 
correctly we may say that the actions of pale muscles are 
either relatively feeble though frequent (as in the massive 
flanks of a fish), or relatively infrequent though strong (as in 
the pectoral muscles of a common fowl) ; while the actions of 
dark muscles are both frequent and strong. Some such dif- 
ferentiation may be anticipated by inference from the respec- 
tive physiological requirements. A muscle which has upon 
occasion to evolve considerable force, but which has thereafter 
a long period of rest during which repair may restore it to 
efficiency, requires neither a large reserve of the contrac- 
tile substance that is in some way deteriorated by action, 
nor highly-developed appliances for bringing it nutri- 
tive materials and removing effete products. Where, con- 
trariwise, an exerted muscle that has undergone much 
molecular change in evolving mechanical force,-has soon again 
to evolve much mechanical force, and so on continually ; it 
is clear that either the quantity of contractile substance 
present must be great, or the apparatus for nutrition and 
depuration must be very efficient, or both. Hence we may 
look for marked unlikenesses of minute structure between 
muscles that are markedly contrasted in activity. And we may 
suspect that these conspicuous contrasts of colour between 
active and inactive muscles, are due to these implied diffe- 
rences of minute structure—partly differences between the 
numbers of blood-vessels and partly differences between the 
quantities of sarcous matter. ; 

Here, then, we have a key to the apparent anomaly above 
hinted at—the maintenance of bulk by certain muscles which 
have been rendered comparatively inactive by changed habits 
of life. That the pectoral muscles of those domestic birds 


THE INNER TISSUES OF ANIMALS. 359° 


which fly but little, have not dwindled to any great extent, 
has been thought a fact at variance with the conclusion that 
functionally-produced adaptations are inheritable. It has 
been argued that if parts which are exercised increase, not 
only in the individual but in the race, while parts which 
become less active decrease ; then a notable difference of size 
should exist between the muscles used for flight in birds that 
fly much, and those in birds of an allied kind that fly little. 
But, as we here see, this is not the true implication. The 
change in such cases: must be chiefly in vascularity and abun- 
dance of contractile substance ; and cannot be, to any great 
extent, in bulk. For a bird to fly at all, its pectoral muscles, 
bones of attachment, and all accompanying appliances, must 
be kept up to-a certain level of power. If the parts dwindle 
much, the creature will be unable to lift itself from the 
ground. Bearing in mind that the force which a bird ex- 
pends to sustain itself in the air during each successive instant 
of a short flight, is, other things equal, the same as it ex- 
pends in each successive instant of a long flight, we shall see 
that the muscles employed in the two cases must have some 
thing like equal intensities of contractile power ; and that the 
structural ditferences between them must have relation mainly 
to the lengths of time during which they can continue to re- 
peat contractions of like intensity. ‘That is to say, while the 
power of flight is retained at all, the muscles and bones can- 
not greatly dwindle; but the dwindling, in birds whose flights 
are short or infrequent or both, will be in the reserve stock 
of the substance that is incapacitated by action, or in the 
appliances that keep the apparatus in repair, or in both. 
Only where, as in the struthious birds, the habit of flight is 
lost, can we expect atrophy of all the parts concerned in 
flight; and here we find it. 

Are such differentiations among the muscles functionally 
produced ? or are they produced by the natural selection of 
variations distinguished as spontaneous? We have, I think, 
good grounds for concluding that they are functionally pro- 


360 PHYSIOLOGICAL DEVELOPMENT. 


duced. We know that in individual men and animals, the 
power of sustained action in muscles is rapidly adaptable to 
the amount of sustained action required. We know that 
being “out of condition,” is usually less shown by the inability 
to put out a violent effort than by the inability to continue 
making violent efforts; and we know that the result of train- 
ing for prize-fights and races, is more shown in the prolonga- 
tion of energy than in the intensification of energy. At the 
same time, experience has taught us that the structural change 
which accompanies this functional change, is not so much a 
change in the bulk of the muscles as a change in their inter- 
nal state: instead of being soft and flabby they become hard. 
We have inductive proof, then, that exercise of a muscle causes 
some interstitial growth along with the power of more sus- 
tained action; and there can be no doubt that the one is a 
condition to the other. What is this interstitial growth? 
There is reason to suspect that it is in part an inereased 
deposit of the sarcous substance and in part a development of 
blood-vessels. | Microscopic observation tends to confirm the 
conclusions before drawn, that repetition of contractions fur- 
thers the formation of the matter which contracts, and that 
greater draughts of blood determine greater vascularity. 
And if the contrasts of molecular structure and the contrasts 
of vascularity, directly caused in muscles by contrasts in their 
activities, are to any degree inheritable; there results an 
explanation of those constitutional differences in the colours 
and textures of muscles, which accompany constitutional 
differences in their degrees of activity. 

It may be added that if we are warranted in so ascribing 
the differentiations of muscles from one another to direct 
equilibration, then we have the more reason for thinking 
that the differentiation of muscles in general from other 
structures is also due to direct equilibration. That unlike- 
nesses between parts of the contractile tissues having unlike 
functions, are caused by the unlikenesses of their functions, 
renders it the more probable that the unlikenesses between 


THE INNER TISSUES OF ANIMALS, 361 


contractile tissue and other tissues, have been caused by ana- 
logous unlikenesses. 


§ 304. These interpretations, which have already occupied 
too large a space, must here be closed. Of course out of 
phenomena so multitudinous and varied, it has been imprac- 
ticable to deal with any but the most important; and it has 
been practicable to deal with these only in a general way. 
Much, however, as remains to be explained, I think the possi- 
bility of tracing, in so many cases, the actions to which these 
internal differentiations may rationally be ascribed, makes it 
likely that the remaining internal differentiations are due to 
kindred actions. We find evidence that in more cases 
than seemed probable, these actions produce their effects 
directly on the individual; and that the unlikenesses are 
produced by accumulation of such effects from generation to 
generation. While for the remaining unlikenesses, we have, 
as an adequate cause, the indirect effects wrought by the sur- 
vival, generation after generation, of the individualsin which 
favourable variations have occurred—variations such as those 
of which human anatomy furnishes endless instances. ‘Thus 
accounting for so much, we may not. unreasonably presume 
that these co-operative processes of direct and indirect equili- 
bration will account for what remains. 

Though not strictly included under the title of the chap- 
ter, there is a subject on which a few words may here be 
added, because of the elucidations yielded to it by some 
parts of the chapter. I refer to the repair and growth of the 
differentiated tissues. When treating inductively of that resto- 
ration which takes place in worn organs, it was admitted that 
little in the way of deductive interpretation is apparent— 
nothing beyond the harmony between the facts and the 
general principle of segregation (§ 64). And it was further 
admitted that it is not obvious why, within certain limits, an 
organ grows in proportion as it is exercised. Certain of the 
foregoing considerations, however, help us towards a partial 


362 PHYSIOLOGICAL DEVELOPMENT. 


rationale of these phenomena. When treating of the de- 
velopment of respiratory surfaces, external or internal, at 
places where the greatest contrast exists between the oxy- 
genated plasma outside the vessels and the carbonized blood 
inside them, reference was made to the truth that the ex- 
change of liquids must, other things equal, be rapid in pro- 
portion as the contrast between them is great. Now this 
truth holds generaily. In every tissue the rate of osmotic 
exchange must vary as this contrast varies; and where the 
contrast is produced by composition or decomposition going 
forward in the tissue, the amount of exchange must be pro- 
portionate to the amount of composition or decomposition. 
If the blood is circulating through an inactive organ, there 
is nothing to disturb, in any great degree, the proximate 
equilibrium between the plasma within the blood-vessels and 
the plasma without them. But if the tissue is functionally 
excited—if it is made to yield up and expend part of the force 
latent in its molecules: or the molecules of the oxy-hydro- 
carbons permeating it, its contained liquid necessarily becomes 
charged with molecules: of another order—simpler molecules ; 
and the greater the amount of function the more different 
is it made from the liquid contained in the blood-vessels. 
Hence the osmotic exchange must be most rapid where the 
metamorphosis of substance is most rapid—the materials for 
consumption and for re-integration of tissue, must be supplied 
in proportion to the demand. ‘This, however, is not the sole 
process by which waste and repair are equilibrated. There 
is the osmotic distension above pointed out as one of the 
causes of circulation—a force tending ever to thrust most 
blood to the places where there is the greatest escape for it ; 
that is—the greatest consumption of it. For since in an active 
tissue, the plasma passing out of its capillaries into its sub- 
stance is continually yielding up its complex molecules, 
either to be assimilated or to be decomposed ; and since-the 
products of decomposition, whether of the nitrogenous tissue 
or of its contained hydro-carbons, are simpler than the 


THE INNER TISSUES OF ANIMALS. 363 


substances from which they arise, and therefore have greater 
molecular mobility; it follows that the liquid contained in 
an active tissue has a greater average molecular mobility 
than the liquids elsewhere; and therefore makes its way 
through the channels of excretion faster than elsewhere: the 
two chief products, carbonic acid and water, escaping with 
especial facility. Hence the place becomes a place of least 
resistance, through which the distended walls of the elastic 
vascular system tend continually to force out an extra 
quantity of plasma. The argument carried a 
step further, yields us an idea of the way in which not only 
repair but also growth of the exercised tissue may be caused 
—at least, where this tissue is one which evolves force. 
Assuming it to be established that the force generated 
by muscle does not result from the consumption of its nitro- 
genous substance, but from the consumption of its contained 
hydro-carbons and oxy-hydro-carbons; and inferring that a 
large amount of muscular action may be performed without 
a corresponding loss of nitrogenous substance; we get a 
clue to the process of increase in a specially-exercised 
muscle. For if osmotic exchange and osmotic distension 
conspire to produce a more rapid passage of plasma out 
of the capillaries into this active tissue than into inactive 
tissues; and if, of the substances in this larger supply of 
plasma, only the non-nitrogenous are consumed; then there 
must be an accumulation of the nitrogenous substances. If 
the waste of the albuminous components of the tissue has 
not kept pace with the consumption of its carbonaceous con- 
tents; then there will exist in the liquid permeating it more 
albuminous substance than is needed for its repair—there 
will be material for its growth. The growth thus resulting, 
however, will be limited both by the capacity of the channels 
of supply and by the competing absorption of other active 
tissues. So long as one muscle, or set of muscles, is 
specially exercised, while the rest discharge but small 
amounts of duty—so long, that is, as the quantity of 


364 PHYSIOLOGICAL DEVELOPMENT. 


tissue-forming matters taken from the alimentary canal into 
the blood, is not largely draughted off elsewhere, this local 
growth may go on. But if many other sets of muscles are 
similarly active, the abstraction of tissue-forming matters at 
various places, will so far diminish their abundance in the 
blood, as to reduce the supply available at any one place for 
growth: eventually leaving sufficient for repair only. 

Though we lack data for thus interpreting specifically 
the repair and growth of other active tissues, yet we may see, 
in a general way, that a parallel interpretation holds. For 
if any tissue that consumes, transforms, excretes, or secretes 
matters that pass into it from the blood, is not formed of the 
same constituents as these matters it transforms or excretes ; 
or if it does not undergo waste proportionate to the quantity 
of matter it transforms or excretes; then it seems fairly 
inferable that along with any unusual quantity of such 
matters to be transformed or excreted, the plasma passing into 
it must bring a surplus of the materials for its own repair 
and growth. 


CHAPTER 1X. 
PHYSIOLOGICAL INTEGRATION IN ANIMALS. 


§ 305. Physiological differentiation and physiological inte- 
gration, are correlatives that vary together. We have but 
to recollect the familiar parallel between the division of 
labour in a society and the physiological division of la- 
bour, to see that as fast as the kinds of work performed by 
the component: parts of an organism become more numerous, 
and as ‘fast as each part becomes more restricted to its own 
work, so fast must the parts have their actions combined in 
such ways that no one can go on without the rest and the 
rest cannot go on without each one. 

Here our inquiry must be, how the relationship of 
these two processes is established—what causes the inte- 
gration to advance pari passu with the differentiation. 
Though it is manifest, d@ priori, that the mutual dependence 
of functions must be proportionate ‘to the specialization of 
functions; yet it remains to find the mode in which the in- 
creasing co-ordination is determined. 

Already, among the Inductions of Biology, this relation 
between differentiation and integration has been specified 
and illustrated (§ 59). Before dealing with it deductively, 
a ‘few further examples, grouped so as to exhibit its several 
aspects, will be advantageous. 


§ 306. If the lowly-organized Planaria has its body 
broken up and its gullet detached, this will, for a while, 


366 PHYSIOLOGICAL DEVELOPMENT. 


continue to perform its function when called upon, just 
as though it were in its place: a fragment of the creature’s 
own body placed in the gullet, will be propelled through it, 
or swallowed by it. But, as the seeming strangeness of this 
fact implies, we find no such independent actions of analogous 
parts in the higher animals. A piece cut out of the 
disc of a Medusa, continues with great persistence repeating 
those rhythmical contractions which we see in the disc as 
a whole; and thus proves to us that the contractile function 
in each portion of the disc, isin great measure independent. 
But it is not so with the locomotive organs of more differen- 
tiated types. When separated from the rest, these lose their 
powers of movement. The only member of a vertebrate animal 
which continues to act after detachment, is the heart; and 
the heart has a motor apparatus complete within itself. 
Where there is this small dependence of each part upon 
the whole, there is but small dependence of the whole 
upon each part. The longer time which it takes for the 
arrest of a function to produce death in a less differentiated 
animal than in a more differentiated animal, may be illus- 
trated by the case of respiration. Suffocation in a man 
speedily causes resistance to the passage of the blood through 
the capillaries, followed by congestion and stoppage of the 
heart: great disturbance throughout the system results in a 
few seconds; and in a minute or two all the functions cease. 
But in a frog, with its undeveloped respiratory organ, and a 
skin through which a considerable aération of the blood is 
carried on, breathing may be suspended for a long time 
without injury. Doubtless this difference is proximately due 
to the greater functional activity in the one case than in the 
other, and the more pressing need for discharging the pro- 
duced carbonic acid; but the greater functional activity being 
itself made possible by the higher specialization of functions, 
this remains the primary cause of the greater dependence of 
the other functions on respiration, where the respiratory 
apparatus has become highly specialized. Here. 


PHYSIOLOGICAL INTEGRATION IN ANIMAIS. 367 


indeed, we see the relation under another aspect. This more 
rapid rhythm of the functions which increased heterogeneity 
of structure makes possible, is itself a means of integrating 
the functions. Watch, when it is running down, a compli- 
cated machine of which the parts are not accurately adjusted, 
or are so worn as to be somewhat loose. There will be 
observed certain irregularities of movement just before it 
comes to rest—certain of the parts which stop first, are 
again made to move a little by the continued movement 
of the rest, and then become themselves, in turn, the 
causes of renewed motion in other parts which have ceased 
to move. That is to say, while the connected rhythmical 
changes of the machine are quick, their actions and re- 
actions on one another are regular—all the motions are well 
integrated ; but as the velocity diminishes, irregularities arise 
—the motions become somewhat disintegrated. Similarly 
with organic functions: increase of their rapidity involves 
increase of a joint momentum which controls each and co- 
ordinates all. Thus, if we compare a Snake with a Mammal, 
we see that its functions are not tied together so closely. 
The Mammal, and especially the superior Mammal, requires 
food with considerable regularity ; keeps up a respiration 
that varies within but moderate limits; and has periods of 
activity and rest that alternate evenly and frequently. But 
the Snake, taking food at long intervals, may have these 
intervals greatly extended without fatal results; its dormant 
and its active states recur less uniformly ; and its rate of 
respiration varies within much wider limits—now being 
scarcely perceptible, and now, as you may prove by exciting 
it, becoming conspicuous. So that here, where the rhythms 
are very slow, they are individually less regular, and are 
united into a less regular compound rhythm—are less in- 
tegrated. 

Perhaps the clearest general idea of the co-ordination of 
functions that accompanies their specialization, is obtained by 
observing the slowness with which a little-differentiated animal 


368 PHYSIOLOGICAL DEVELOPMENT. 


responds to a stimulus applied to one of its parts, and the 
rapidity with which such a local stimulus is responded to by 
a more-differentiated animal. A Polype and a Polyzoon, two 
creatures somewhat similar in their outward appearances but 
very unlike in their internal structures, will serve for the 
comparison. A tentacle of a Polype, when touched, slowly 
contracts; and if the touch has been rude, the contraction 
presently extends to the other tentacles and eventually to the 
entire body: the stimulus to movement is gradually diffused 
throughout the organism. But if you touch a tentacle of a 
Polyzoon, or slightly disturb the water near it, the whole 
cluster of tentacles is instantly withdrawn, along with the 
protruded part of the creature’s body, into its sheath. Whence 
arises this contrast? The one creature has no specialized 
contractile organs, or fibres for conveying impressions. ‘The 
other has definite muscles and nerves. The parts of the 
little-differentiated Polype have their functions so feebly co- 
ordinated, that one may be strongly affected for a long time 
before any effect is felt by another at a distance from it; but 
in the more-differentiated Polyzoon, various remote parts 
instantly have changes propagated to them from the affected 
part, and by their united actions thus set up, the whole 
organism adjusts itself so as to avoid the danger. — 

These few added illustrations will make the nature of this 
general relation sufficiently clear. Let us now pass to the 
interpretation of it. 


§ 307. If a Hydra is cut in two, the nutritive liquids 
diffused through its substance cannot escape rapidly, since 
there are no open channels for them; and hence the condi- 
tion of the parts at a distance from the cut is but little 
affected. But where, as in the more-differentiated animals, 
the nutritive liquid is contained in vessels that have con- 
tinuous communications, cutting the body in two, or cutting 
eff any considerable portion of it, is followed by escape of 
the liquid from these vessels to a large extent; and this 


PHYSIOLOGICAL INTEGRATION IN ANIMALS. 569 


affects the nutrition and efficiency of organs remote from 
the place of injury. Then where, as in further-developed 
creatures, there exists an apparatus for propelling the blood 
through these ramifying channels, injury of a single one 
will cause a loss of blood that quickly prostrates the entire 
organism. Hence the rise of a completely-differentiated vas- 
cular system, is the rise of a system which integrates all 
members of the body, by making each dependent on the in- 
tegrity of the vascular system, and therefore on the integrity 
of each member through which it ramifies. In 
another mode, too, the establishment of a distributing 
apparatus produces a physiological union that is great in 
proportion as this distributing apparatus is efficient. As 
fast as it assumes a function unlike the rest, each part of an 
animal modifies the blood in a way more or less unlike the 
rest, both by the materials it abstracts and by the products it 
adds; and hence the more differentiated the vascular system 
becomes, the more does it integrate all parts by making each 
of them feel the qualitative modification of the blood which 
every other has produced. This is simply and conspicuously 
exemplified by the lungs. In the absence of a vascular 
system, or in the absence of one that is well marked off 
from the imbedding tissues, the nutritive plasma or the crude 
blood, gets what small aération it can, only by coming near 
the creature’s outer surface, or those inner surfaces that are 
bathed by water; and it is probably more by osmotic ex- 
change than in any other way, that the oxygenated plasma 
slowly permeates the tissues. But where there have been 
formed definite channels branching throughout the body; 
and particularly where there exist specialized organs for 
pumping the blood through these channels; it manifestly 
becomes possible for the aération to be carried on in one part 
peculiarly modified to further it, while all other parts have 
the aérated blood brought to them. And how greatly the 
differentiation of the vascular system thus becomes a means 
of integrating the various organs, is shown by the fatal 
VOL. I. 24 


370 PHYSIOLOGICAL DEVELOPMENT. 


result that follows when the current of aérated blood is 
interrupted. | 
Here, indeed, it becomes obvious both that certain physio- 
logical differentiations make possible certain physiological 
integrations ; and that, conversely, these integrations make 
possible other differentiations. Besides the waste products 
that escape through the lungs, there are waste products that 
escape through the skin, the kidneys, the liver. The blood 
has separated from it in each of these structures, the par- 
ticular product, which this structure has become adapted to 
separate ; leaving the other products to be separated by the 
other adapted structures. How have these special adaptations 
been made possible? By union of the organs as recipients of 
one circulating mass of blood. While there is no eflicient 
apparatus for transfer of materials through the body, the 
waste products of each part have to make their escape locally; 
and the local channels of escape must be competent to. take 
off indifferently all the waste products. But it becomes prac- 
ticable and advantageous for the differently-localized ex- 
- ereting structures, to become fitted to separate different waste 
products, as soon as the common circulation through them 
erows so efficient that the product left unexcreted by one is 
quickly carried to another better fitted to excrete it. So that 
the integration of them through a common vascular system, 
is the condition under which only they can become differen- 
tiated. How the specialization of each is rendered possible 
only by its connexion with others that have become similarly 
specialized, we indirectly see in such a fact as that in chronic 
jaundice secondary disease of the kidneys is apt to arise in 
consequence of the biliverdine accumulated in the system 
being partly excreted through them: the implication being 
that a structure peculiarly fitted to excrete urea can exist 
only when it is functionally united with another structure 
peculiarly fitted to excrete biliverdine. Perhaps the 
clearest idea of the way in which differentiation leads to 
integration, and how, again, increased integration makes 


PHYSIOLOGICAL INTEGRATION IN ANIMALS. 37h 


possible still further differentiation, will be obtained by con- 
templating the analogous dependence in the social organism. 
While it has no roads, a country cannot have its industries 
much specialized: each locality must produce, as best it can, 
the various commodities it consumes, so long as it has no 
facilities for barter with other localities. But the localities 
being unlike in their natural fitnesses for the various indus- 
tries, there tends ever to arise some exchange of the commo- 
dities they can respectively produce with least labour. This 
exchange leads to the formation of channels of communica- 
tion. ‘The currents of commodities once set up, make their 
foot-paths and horse-tracks more permeable; and as fast as 
the resistance to exchange becomes less, the currents of 
commodities become greater. Hach locality takes more 
of the products of adjacent ones, and each locality devotes 
itself more to the particular industry for which it is naturally 
best fitted: the functional integration makes possible a further 
functional differentiation. This further functional differen- 
tiation reacts. The greater demand for the special product of 
each locality, excites improvements in production—leads to 
the use of methods which both cheapen and perfect the com- 
modity. Hence results a still more active exchange; a still 
clearer opening of the channels of communication; a still 
closer mutual dependence. Yet another influence comes into 
play. As fast as the intercourse, at first only between neigh- 
bouring localities, makes for itself better roads—as fast as 
rivers are bridged and marshes made easily passable, the 
resistance to distribution becomes so far diminished, that the 
things grown or made in eachdistrict can be profitably carried 
to a greater distance; and as the economical integration is 
thus extended over a wider area, the economical differentia- 
tion is again increased ; since each district, having. a larger 
market for its commodity, is led to devote itself more exclu- 
sively to producing this commodity. These actions and re- 
actions continue until the various localities, becoming greatly 
developed and highly specialized in their industries, are at 


7g ied 


3i2 PHYSIOLOGICAL DEVELOPMENT. 


the same time functionally integrated by a network of roads, 
and finally railways, along which rapidly circulate the cur- 
rents severally sent out and received by the localities. And 
it will be manifest that in individual organisms a like corre- 
‘ative progress must have been caused in an analogous way. 


§ 308. Another and higher form of physiological integra- 
tion in animals, is that which the nervous system effects. 
Each part as it becomes specialized, begins to act upon the 
rest. not only indirectly through the matters it takes from 
and adds to the blood, but also directly through the molecular 
disturbances it sets up and diffuses. Whether nerves them- 
selves are differentiated by the molecular disturbances thus 
propagated in certain directions, or whether they are other- 
wise differentiated, it must equally happen that as fast as 
they become channels along which molecular disturbances 
travel, the parts they connect become physiologically in- 
tegrated, in so far that a change in one initiates a change in 
the other. We may dimly perceive that if portions of what 
was originally a uniform mass having a common function, 
undertake sub-divisions of the function, the molecular 
changes going on in them will be in some way complemen- 
tary to one another: that peculiar form of molecular motion 
which the one has lost in becoming specialized, the other has 
gained in becoming specialized. And if the molecular motion 
that was common to the two portions while they were undiffer- 
entiated, becomes divided into two complementary kinds of 
molecular motion; then between these portions there will be a 
contrast of molecular motions such that whatever is plus in 
the one will be minus in the other ; and hence there will be a 
special tendency towards a restoration of the molecular equili- 
brium between the two: the molecular motion continually 
propagated away from either will have its line of least resist- 
ance in the direction of the other. If, as argued 
in the last chapter, repeated restorations of molecular equili- 
brium, always following the line of least resistance, tend ever 


PHYSIOLOGICAL INTEGRATION IN ANIMALS 373 


to make it a line of diminished resistance ; then, in propor- 
tion as any parts become more physiologically integrated by 
the establishment of this channel for the easy transmission 
of molecular motion between them, they may become more 
physiologically differentiated. The contrast between their 
molecular motions leads to the line of discharge; the line of 
discharge, once formed, permits a greater contrast of their 
‘molecular motions to arise; thereupon the quantities of 
molecular motion transferred to restore equilibrium, being 
increased, the channel of transfer is made more permeable ; 
and its further permeability, so caused, renders possibile a still 
more marked unlikeness of action between the parts. Thus 
the differentiation and the integration progress hand in hand 
as before. How the same principle holds through- 
out the higher stages of nervous development, can be seen 
only still more vaguely. Nevertheless, it is comprehensible 
that as functions become further divided, there will arise the 
need for sub-connexions along which there may take place 
secondary equilibrations subordinate to the main ones. It is 
manifest, too, that whereas the differentiation of functions 
proceeds, not necessarily by division into two, but often by 
division into several, and usually in such ways as not to leave 
any two functions that are just complementary to one another, 
the restorations of equilibrium cannot be so simple as 
above supposed. And especially when we bear in mind that 
many differentiated functions, as those of the senses, cannot 
be held complementary to any other functions in particular ; 
it becomes manifest that the equilibrations that have to be 
made in an organism of much heterogeneity, are extremely 
complex, and do not take place between each organ and some 
other, but between each organ and allthe others. The pecu- 
liarity of the molecular motion propagated from each organ, 
has to be neutralized by some counter-peculiarity in the 
average of the molecular motions with which it is brought 
into relation. All the variously-modified molecular motions 
from the various parts, must have their pluses and minuses 


374 PHYSIOLOGICAL DEVELOPMENT. 


mutually cancelled: if not locally, then at some centre to 
which each unbalanced motion travels until it meets with 
some opposite unbalanced motion to destroy it. Still, involved 
as these actions must become, it is possible to see how the 
general principle illustrated by the simple case above sup- 
posed, will continue to hold. For always the molecular 
motion proceeding from any one differentiated part, will travel 
most readily towards that place where a molecular motion 
most complementary to it in kind exists—no matter whether 
this complementary molecular motion be that proceeding 
from any one other organ, or the resultant of the molecular 
motions proceeding from many other organs. So that the 
tendency will be for each channel of communication or nerve, 
to unite itself with some centre or ganglion, where it comes 
into relation with other nerves. And if there be any parts 
of its peculiar molecular motion uncancelled by the mole- 
cular motions it meets at this centre; or if, as will pro- 
bably happen, the average molecular motion which it there 
unites to produce, differs from the average molecular motion 
elsewhere ; then, as before, there will arise a discharge along 
another channel or nerve to another centre or ganglion, where 
the residuary difference may be cancelled by the differences 
it meets; or from whence it may be still further propagated 
till it is so cancelled. Thus there will be a tendency to a 
general nervous integration keeping pace with the differen- 
tiation. 

Of course this must be taken as nothing more than the 
indication of initial tendencies—not as an hypothesis suffi- 
cient to account for all the facts. It leaves out of sight the 
origin and functions of ganglia, considered as something 
more than nerve-junctions. Were there only these lines of 
easy transmission of molecular disturbance, a change set up 


in one organ could never do more than produce its equivalent | 


of change in some other or others; and there could be none 
of that large umount of motion initiated by a small sensation, 
which we habitually see. The facts show, unmistakably, that 


PHYSIOLOGICAL INTEGRATION IN ANIMALS. 375 


the slight disturbance communicated to a ganglion, causes an 
overthrow of that highly-unstable nervous matter contained 
in it, and a discharge from it of the greatly-increased quantity 
of molecular motion so generated. This, however, is beyond 
our immediate topic. All we have here to note is the inter- 
dependence and unification of functions that naturally follow 
the differentiation of them. 


§ 309. Something might be added concerning the 
further class of integrations by which organisms are con- 
stituted mechanically-coherent wholes. Carrying further 
certain of the arguments contained in the last chapter, it 
might be not unreasonably inferred that the binding together 
of parts by bones, muscles, and ligaments, is a secondary result 
of those same actions by which bones, muscles, and ligaments 
are specialized. But adequate treatment of this division of 
the subject is at present scarcely possible. 

What little of fact and inference has been above set down, 
will, however, serve to make comprehensible the general truths 
respecting which, in their main outlines, there can be no 
question. Beginning with the feebly-differentiated sponge, 
of which the integration is also so feeble that cutting off a 
piece interferes in no appreciable degree with the activity 
and growth of the rest, it is undeniable that the advance 
is through stages in which the multiplication of unlike parts 
having unlike actions, is accompanied by an increasing inter- 
dependence of the parts and their actions; until we come to 
structures like our own, in which a slight change initiated in 
one part will instantly and powerfully affect all other parts— 
will convulse an immense number of muscles, send a wave of 
contraction through all the blood-vessels, awaken a crowd of 


ideas with an accompanying gush of emotions, affect the 


action of the lungs, of the stomach, and of all the secreting 
organs. And while it is a manifest necessity that along with 
this subdivision of functions which the higher organisms show 


us, there must be this close co-ordination of them, the fore-- 


r 


3/76 _ PHYSIOLOGICAL DEVELOPMENT. 


going paragraphs suggest how this necessary correlation is 
brought about. For a great part of the physiological union 
that accompanies the physiological specialization, there 
appears to be a sufficient cause in the process of direct equili- 
bration ; and indirect equilibration may be fairly presumed a 
sufficient cause for that which remains. 


CHAPTER X. 
SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. 


§ 310. Intercourse between each part and the particular 
‘conditions to which it is exposed, either habitually in the 
individual or occasionally in the race, thus appears to be the 
origin of physiological development; as we found it to be the 
origin of morphological development. The unlikenesses of 
‘form that arise among members of an aggregate that were 
originally alike, we traced to unlikenesses in the incident forces, 
And in the foregoing chapters we have traced to unlikenesses 
in the incident forces, those unlikenesses of minute structure 
and chemical composition that simultaneously arise among: 
the parts. | 
- In summing up the special truths illustrative of this 
general truth, it will be proper here to contemplate more 
especially their dependence on first principles. Dealing with 
biological phenomena as phenomena of evolution, we have to 
interpret not only the increasing morphological heterogeneity 
of organisms, but also their increasing physiological hetero- 
geneity, in terms of the re-distribution of matter and motion 
While we make our rapid re-survey of the facts, let us then 
more particularly observe how they are subordinate to the 
universal course of this re-distribution. 


_ § 311. The instability of the homogeneous, or, strictly 
speaking, the inevitable lapse of the more homogeneous into 
the less homogeneous, which we before saw endlessly exem- 


378 PHYSIOLOGICAL DEVELOPMENT. 


plified by the morphological differentiations of the parts of 
organisms, we have here seen afresh exemplified in ways also 
countless, by the physiological differentiations of their parts. 
And in the one case as in the other, this change from uni- 
formity into multiformity in organic aggregates, 1s caused, as 
it is in all inorganic aggregates, by the necessary exposure 
of their component parts to actions unlike in kind or quan- 
tity or both. General proof of this is furnished by the order 
in which the differences appear. if parts are rendered 
physiologically heterogeneous by the heterogeneity of the 
incident forces; then the earliest contrasts should be between 
parts that are the most strongly contrasted in their relations 
to incident forces; the next earliest contrasts should occur 
where there are the next strongest contrasts in these relations ; 
and soon. It turns out that they do this. 

Everywhere the differentiation of outside from inside 
comes first. In the simplest plants the unlikeness of 
the cell-wall to the cell-contents is the conspicuous trait of 
structure. The contrasts seen in the simplest animals are 
of the same kind: the film that covers a Rhizopod and the 
more indurated coat of an Jnfusorium, are more unlike the 
contained sarcode than the other parts of this are from one 
another; and the tendency during the life of the animal is 
for the unlikeness to become greater. What is true 
of Protophyta and Protozoa, is true of the germs of all organ- 
isms up to the highest: the differentiation of outer from inner 
is the first step. When the endochrome of an Alga-cell has 
broken up into the clusters of granules which are eventually 
to become spores, each of these quickly acquires a mem- 
branous coating ; constituting an unlikeness between surface 
and centre. Similarly with the ovule of every higher plant: 
the mass of cells forming it, early exhibits an outside layer of 
cells distinguished from the cells within. With animal germs 
it is the same. Je it in a ciliated gemmule, be it in the 
pseud-ova of Aphides and of the Cecidomyia, or be it in 
true ova, the primary differentiation conforms to the relations 


SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. 379 


of exterior and interior. If we turn to adult or- 
ganisms, vegetal or animal, we see that whether they do or 
do not display other contrasts of parts, they always display 
this contrast. Though otherwise almost homogeneous, such 
Fungi as the Puff-ball, or, among Alga, all which have a 
thallus of any thickness, present marked differences between 
those of their cells which are in immediate contact with the 
environment and those which are not. Such differences they 
present in common with every higher plant; which, 
here in the shape of bark and there in the shape of 
cuticle, has an envelope inclosing it even up to its petals : 
the only parts not so inclosed, being those short-lived 
terminations of the fructifying organs, from which the dis- 
integrated tissue 1s being cast off to form the germs of new 
individuals. In like manner among animals, there is always 
either a true skin or an outer coat analogous to one. Wher- 
ever aggregates of the first order have united into ag- 
gregates of the second and third orders—wherever they 
have become the morphological units of such higher aggre- 
gates—the outermost of them have grown unlike those lying 
within. Even the Sponge is not without a layer that may 
by analogy be called dermal. 

This lapse of the relatively homogeneous into the rela- 
tively heterogeneous, first showing itself, as on the hypothesis 
of evolution it must do, by the rise of an unlikeness between 
outside and inside, goes on next to show itself, as we infer 
that it must do, by the establishment of secondary contrasts 
among the outer parts answering to secondary contrasts 
among the forces falling on them. So long as the whole sur- 
face of a plant remains similarly related to the environment, 
as in a Protococcus or a Volvox, it remains uniform ; but when 
there come to be an attached surface and a free surface, 
these, being subject to unlike actions, are rendered unlike. 
This is visible even in a unicellular A/ga when it becomes 
fixed; it is shown in the distinction between the under 
and upper parts of ordinary Fungi; and we see it i 


380 PHYSIOLOGICAL DEVELOPMENT. 


the universal difference between the imbedded ends and the 
exposed ends of the higher plants. And then among the 
less marked contrasts of surface answering to the less marked 
contrasts in the incident forces, come those between the 
upper and under sides of leaves; which, as we have seen, 
vary in degree as the contrasts of forces vary in degree, and 
disappear where these contrasts disappear. Equally 
clear proof is furnished by animals, that the original uni- 
formity of surface lapses into multiformity, in proportion as 
the actions of the environment upon the surface become 
multiform. In a Worm, burrowing through damp soil that 
acts equally on all its sides, or in a Tenia, uniformly bathed 
by the contents of the intestine it inhabits, the parts of the 
integument do not appreciably differ from one another ; but 
in creatures not surrounded by the same agencies, as those 
that crawl and those that have their bodies partially inclosed, 
there are unlikenesses of integument corresponding to unlike- 
nesses of the conditions. A Snail’s foot has an under 
surface not uniform with the exposed surface of its body, and 
this again is not uniform with the protected surface. Among 
articulate animals there is usually a distinction between the 
ventral and the dorsal aspects; and in those of the Articulata 
‘which subject their anterior and posterior ends to different 
environing agencies, as do the Ant-lion and the Hermit-crab, 
these become superficially differentiated. Ana- 
logous general contrasts occur among the Vertebrata. Fish, 
though their outsides are uniformly bathed by water, have 
their backs more exposed to light than their bellies; and the 
two are commonly distinct in colour. Where it is not the 
‘back and belly that are thus dissimilarly conditioned, but the 
sides, as in the Pleuronectide, then it is the sides that be- 
come contrasted ; arid there may be significance in the fact, 
that those abnormal individuals of this order which revert to 
the ancestral undistorted type, and swim vertically, have the 
two sides alike. In such higher vertebrates as Reptiles, we 
‘see repeated this differentiation of the upper and under sur- 


SUMMARY OF PHYSIOLOGICAL DEVELOPMENT, 381 


faces: especially in those of them which, like Snakes, ex- 
pose these surfaces to the most diverse actions. Even in 
Birds and Mammals which usually, by raising the under 
surface considerably above the ground, greatly diminish the 
contrast between its conditions and the conditions to which 
the upper surface is subject, there still remains some unlike- 
ness of clothing answering to the remaining unlikeness be- 
tween the conditions. Thus, without by any 
means saying that all such differentiations are directly 
caused by differences in the actions of incident forces, which, 
as before shown (§ 294), they cannot be, it is clear that 
many of them are so caused. It is clear that parts of the 
‘surface exposed to very unlike environing agencies, become 
very unlike; and this is all that needs be shown. 

Complex as are the transformations of the inner parts of 
organisms from the relatively homogeneous into the rela- 
tively heterogeneous, we still see among them a conformity 
to the same general order. In both plants and animals the 
earlier internal differentiations answer to the stronger con- 
trasts of conditions. Plants, absorbing all their 
nutriment through their outer surfaces, are internally modi- 
fied mainly by the transfer of materials and by mechanical 
stress. Such of them as do not raise their fronds above the 
surface, have their inner tissues subject to no marked con- 
trasts save those caused by currents of sap; and the lines of 
lengthened and otherwise changed cells that are formed 
where these currents run, and are most conspicuous where 
these currents must obviously be the strongest, are the only 
decided differentiations of the interior. But where, as in 
the higher Oryptogams and in Phenogams, the leaves are 
upheld, and the supporting stem is transversely bent by 
the wind, the inner tissues, subject to different amounts of 
mechanical strain, differentiate accordingly: the deposit of 
dense substance commences in that region where the sap- 
containing cells and canals suffer the greatest intermittent 
éompressions. | Animals, or at least such of them 


3882 PHYSIOLOGICAL DEVELOPMENT. 


as take food into their interiors, are subject to forces of 
another class tending to destroy their original homogeneity. 
Food is a foreign substance which acts on the interior as an 
environing object which touches it acts on the exterior—is 
literally a portion of the environment, which, when swal- 
lowed, becomes a cause of internal differentiations as the rest 
of the environment continues a cause of external differentia- 
tions. How essentially parallel are the two sets of actions 
and reactions, we have seen implied by the primordial identity 
of the endoderm and ectoderm in simple animals, and of the 
skin and mucous membrane in complex animals (§§ 288, 289). 
Here we have further to observe that as food is the original 
source of internal differentiations, these may be expected to 
show themselves first where the influence of the food is 
greatest ; and to appear later in proportion as the parts are 
more removed from the influence of the food. They do this. 
Tn animals of low type, the coats of the alimentary cavity or 
canal, are more differentiated than the tissue that lies between 
the alimentary canal and the wall of the body. This tissue 
in the higher Coelenterata, is a feebly-organized parenchyma 
traversed by lacunze—either simple channels, or canals lined 
with simple ciliated cells; and in the lower Mollusca the 
structures bounding the perivisceral cavity and its ramifying 
sinuses, are similarly imperfect. Further, it is observable 
that the differentiation of this perivisceral sac and its sinuses 
into a vascular system, proceeds centrifugally from the 
region where the absorbed nutriment enters the mass of cir- 
culating liquid, and where this liquid is qualitatively more 
unlike the tissues than it is at the remoter parts of the body. 

Physiological development, then, is initiated by that insta- . 
bility of the homogeneous which we have seen to be every- 
where a cause of evolution (first Principles, 8§ 109—115). That 
the passage from comparative uniformity of composition and 
minute structure to comparative multiformity, is set up in 
organic aggregates, as in all other aggregates, by the neces- 
sary unlikenesses of the actions to which the parts are sub- 


SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. 389 


ject, is shown by the universal rise of the primary differentia- 
tion between the parts that are universally most contrasted 
in their circumstances, and by the rise of secondary differen- 
tiations obviously related in their order to secondary contrasts 
of conditions. 


§ 312. How physiological development has all along been 
aided by the multiplication of effects—how each differen- 
tiation has ever tended to become the parent of new differen- 
tiations, we have had, incidentally, various illustrations. Let 
us here review the working of this cause. 

Among plants we see it in the production of progressively- 
multiplying heterogeneities of tissue by progressive increase 
of bulk. The integration of fronds into axes and of axes into 
groups of axes, sets up unlikenesses of action among the in- 
tegrated units, followed by unlikenesses of minute structure. 
Each gust transversely strains the various parts of the stem 
in various degrees, and longitudinally strains in various degrees 
the roots ; and while there is inequality of stress at every place 
in stem and branch, so, at every place in stem and branch, the 
outer layers and the successively inner layers are severally 
extended and compressed to unequal amounts, and have un- 
equal modifications wrought in them. Let the tree add to its 
periphery another generation of the units composing it, and 
immediately the mechanical strains on the supporting parts 
are all changed in different degrees, initiating new differences 
internally. Externally, too, new differences are initiated. 
Shaded by the leaf-bearing outer stratum of shoots, the inner 
structures cease to bear leaves, or to put out shoots that 
bear leaves; and instead of that green covering which 
they originally had, become covered with bark of increasing 
thickness. Manifestly, then, the larger integration of units 
that are originally simple and uniform, entails physiological 
changes of various orders, varying in their degrees at all 
parts of the aggregate. Hach branch which, favourably cir- 
cumstanced, flourishes more than its neighbours, becomes a 


384 PHYSIOLOGICAL DEVELOPMENT. 


cause of physiological differentiations, not only in its neigh- 
bours from which it abstracts sap and presently turns from 
leaf-bearers into fruit-bearers, but also in the remoter parts. 

That among animals physiological development is fur- 
thered by the multiplication of effects, we have lately seen 
proved by the many changes in other organs, which the 
growth or modification of each excreting and secreting 
organ initiates. By the abstracted as well as by the added 
materials, it alters the quality of the blood passing through 
all members of the body; or by the liquid it pours into the 
alimentary canal, it acts on the food, and through it on the 
blood, and through it on the system as a whole: an addi- 
tional differentiation in one part thus setting up additional 
differentiations in many other parts; from each of which, 
again, secondary differentiating forces reverberate through - 
the organism. Or, to take an influence of another order, we 
have seen how the modified mechanical action of any member 
not only modifies that member, but becomes, by its reactions, 
a cause of secondary modifications—how, for example, the 
burrowing habits of the common Mole, leading to an almost 
exclusive use of the fore limbs, have entailed a dwindling 
of the hind limbs, and a concomitant dwindling of the 
pelvis, which, becoming too small for the passage of the 
young, has initiated still more anomalous modifications. 

So that throughout physiological development, as in 
evolution at large, the multiplication of effects has been 
a factor constantly at work, and working more actively 
as the development has advanced. The secondary changes 
wrought by each primary change, have necessarily become 
more numerous in proportion as organisms have become 
more complex. And every increased multiplication of effects, 
further differentiating the organism and, by consequence, 
further integrating it, has prepared the way for still higher: 
differentiations and integrations similarly caused. 


§ 318. The general truth next to be resumed, is that these. 


) 


SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. 385 


processes have for their limit a state of equilibrium —proxi- 
mately a moving equilibrium and ultimately a complete equili- 
brium. ‘The changes we have contemplated are but the con- 
comitants of a progressing equilibration. In every aggregate 
which we call living, as well as in all other aggregates, the 
instability of the homogeneous is but another name for the 
absence of balance between the incident forces and the forces 
which the aggregate opposes to them; and the passage into 
heterogeneity is the passage towards a state of balance. And 
to say that in every aggregate, organic or other, there goes 
on a multiplication of effects, is but to say that one part which. 
has a fresh force impressed on it, must go on changing and 
communicating secondary changes, until the whole of the 
impressed force has been used up in generating equivalent 
reactive forces. 

The principle that whatever new action an organism is 
subject to, must either overthrow the moving equilibrium of 
its functions and cause the sudden equilibration called death, 
or else must progressively alter the organic rhythms, until, 
by the establishment of a new reaction balancing the new 
action, a new moving equilibrium is produced, applies as 
much to each member of an organism as to the organism in 
its totality. Any force falling on any part not adapted to 
bear it, must either cause local destruction of tissue, or must, 
without destroying the tissue, continue to change it until it 
can change it no further; that is—until the modified reaction 
of the part has become equal to the modified action. What- 
ever the nature of the force, this must happen. If it is a 
mechanical force, then the immediate effect is some distortion 
of the part—a distortion having for its limit that attitude 
in which the resistance of the structures to further change of 
position, balances the force tending to produce the further 
change ; and the ultimate effect, supposing the force to be con- 
tinuous or recurrent, is such a permanent alteration of form, 
or alteration of structure, or both, as establishes a permanent 


balance. If the force is physico-chemical, or chemical, the . 
VOL. IL. 25 


386 PHYSIOLOGICAL DEVELOPMENT. 


general result is still the same: the component molecules of 
the tissue must have their molecular arrangements changed, 
and the change in their molecular arrangements must go on 
until their molecular motions are so re-adjusted as to equili- 
brate the molecular motions of the new physico-chemical or 
chemical agent. In other words, the organic matter com- 
posing the part, if it continues to be organic matter at all, 
must assume that molecular composition which enables it to 
bear, or as we say adapts it to, the incident forces. 

Nor is it less certain that throughout the organism as a 
whole, equilibration is alike the proximate limit of the changes 
wrought by each action, as well as the ultimate limit of the 
changes wrought by any recurrent actions or continuous 
action. ‘The ordinary movements every instant going on, are 
movements towards a new state of equilibrium. Raising a 
limb causes a simultaneous shifting of the centre of gravity, 
and such altered tensions and pressures throughout the body 
as re-adjust the disturbed balance. Passage of liquid into or 
out of a tissue, implies some excess of force in one direction 
there at work ; and ceases only when the force so diminishes or 
the counter-forces so increase that the excess disappears. A 
nervous discharge is reflected and re-reflected from part to 
part, until it has all been used up in the re-arrangements pro- 
duced—equilibrated by the reactions called out. And what 
is thus obviously true of every normal change, is equally true 
of every abnormal change—every disturbance of the estab- 
lished rhythm of the functions. If such disturbance is a 
single one, the perturbations set up by it, reverberating 
throughout the system, leave its moving equilibrium slightly 
altered. If the disturbance is repeated or persistent, its suc- 
cessive effects accumulate until they have produced a new 
moving equilibrium adjusted to the new force. 

Each re-balancing of actions, having for its necessary con- 
comitant a modification of tissues, it is an obvious corollary 
that organisms subjected to successive changes of conditions, 
must undergo successive differentiations and re-differentia- 


SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. teva 


tions. Direct equilibration in organisms, with all its accom- 
panying structural alterations, is as certain as is that uni- 
versal progress towards equilibrium of which it forms part. 
_ And just as certain is that indirect equilibration in organisms 
to which the remaining large class of differentiations is due. 
. The development of favourable variations by the killing of 
_ individuals in which they do not occur or are least marked, 
is, as before, a balancing between certain lecal structures and 
the forces they are exposed to; and is no less inevitable than 
the other. 


§ 314. In all which universal laws, we find ourselves again 
brought down to the persistence of. force, as the deepest 
knowable cause of those modifications which constitute 
physiological development; as it is the deepest knowable 
cause of all other evolution. Here, as elsewhere, the per- 
petual lapse from less to greater heterogeneity, the perpetual 
begetting of secondary modifications by each primary modi- 
fication, and the perpetual approach to a temporary balance 
on the way towards a final balance, are necessary implica- 
tions of the ultimate fact that force cannot disappear, but 
can only change its form. 

It is an unquestionable deduction from the persistence of 
force, that in every individual organism each new incident 
force must work its equivalent of change; and that where it 
is a constant or recurrent force, the limit of the change it 
works must be an adaptation of structure such as opposes to 
the new outer force an equal inner force. The only thing 
open to question is, whether such re-adjustment is inherit- 
able; and further consideration will, I think, show, that to 
say it is not inheritable is indirectly to say that force does 
not persist. If all parts of an organism have their fune- 
tions co-ordinated into a moving equilibrium, such that every 
part perpetually influences all other parts, and cannot be 
changed without initiating changes in all other parts—if the 


limit of change is the establishment of a complete harmony 
25 2 


388 . PHYSIOLOGICAL DEVELOPMENT. 


among the movements, molecular and other, of all parts; then 
among other parts that are modified, molecularly or other- 
wise, must be those which cast off the germs of new 
organisms. The molecules of their produced germs must 
tend ever to conform the motions of their components, and 
therefore the arrangements of their components, to the 
molecular forces of the organism as a whole; and if this 
ageregate of molecular forces is modified in its distribution 
by a local change of structure, the molecules of the germs 
must be gradually changed in the motions and arrangements 
of their components, until they are re-adjusted to the aggre- 
gate of molecular forces. Jor to hold that the moving equi- 
librium of an organism may be altered without altering the 
movements going on in a particular part of it, is to hold that 
these movements will net be affected by the altered distribu- 
tion of forces; and to hold this is to deny the persistence of 
force. : 


PART VI. 
LAWS OF MULTIPLICATION. 


QHAPTER I. 
THE FACTORS.* 


§ 315. If organisms have been evolved, their respective 
powers of multiplication must have been determined by 
natural causes. Grant that the countless specialities of 
structure and function in plants and animals, have arisen 
from the actions and reactions between them and their 
environments, continued from generation to generation ; and 
it follows that from these actions and reactions have also 
arisen those countless degrees of fertility which we see 
among them. As in all other respects an adaptation of each 
species to its conditions of existence is directly or indirectly 
brought about; so must there be directly or indirectly 
brought about an adaptation of its reproductive activity to 
its conditions of existence. 

We may expect to find, too, that permanent and temporary 
differences of fertility have the same general interpretation. 
If the small variations of structure and function that arise 
within the limits of each species, are due to actions like those 


* An outline of the doctrine set forth in the following chapters, was 
originally published in the Westminster Review for April, 1852, under the 
title of, A Theory of Population deduced from the General Law of Animal 
Fertility ; and was shortly afterwards republished with a prefatory note, to 
the effect that it must be accepted as a sketch which I hoped at some future 
time to elaborate. 1n now revising and completing it, I have omitted a non- 
essential part of the argument, while I have expanded the remainder by 
adding to the number of facts put in evidence, by meeting objections which 
want of space before obliged me to pass over, and by drawing various 
secondary conclusions. 


892 LAWS OF MULTIPLICATION. 


which, by their long-accumulating effects, have produced the 
immense contrasts between the various types; we may con- 
clude that, similarly, the actions to which changes in the 
rate of multiplication of each species are due, also produce, 
in great periods of time, the enormous differences between 
the rates of multiplication of different species. 

Before inquiring in what ways the rapidities of increase are 
adjusted to the requirements, both temporary and permanent, 
it will be needful to look at the factors. et us set down 
first those which belong to the environment, and then those 
‘which belong to the organism. 


§ 316. Every living aggregate being one of which the 
inner actions are adjusted to balance outer actions, it follows 
that the maintenance of its moving equilibrium depends on 
its exposure to the right amounts of these actions. Its 
“moving equilibrium may be overturned if one of these actions 
is either too great or too small in amount; and it may be so 
overturned either by excess or defect of some inorganic 
agency in its environment, or by excess or defect of some 
organic agency. 

‘Thus a plant, constitutionally fitted to a certain warmth 
and humidity, is killed by extremes of temperature, as well 
as by extremes of drought and moisture. It may dwindle 
away from want of soil, or die from the presence of too great 
or too small a quantity of some mineral substance which the 
soil supplies to it. In lke manner, every animal can main- 
tain the balance of its functions so long only as the environ- 
ment adds to or deducts from its heat at rates not exceeding 
definite limits. Water, too, must be accessible in amount 
sufficient to compensate its loss: if the parched air is rapidly 
abstracting its liquid which there is no pool or river to 
restore, its functions cease ; and if it is an aquatic creature, 
drought may kill it either by drying up its medium or by 
giving it a medium inadequately aérated. Thus each organ- 
ism, adjusted to a certain average in the actions of its 


THE FACTORS. 393 


inorganic environment, or rather, we should say, adjusted to 
certain. moderate deviations from this average, is destroyed 
by extreme deviations. So, too, is it with the 
environing organic agencies. Among plants, only the para- 
sitic kinds depend for their. individual preservation on the 
presence of certain other organisms (though the presence of 
certain other organisms is needful to most plants for the 
preservation of the race by aiding fertilization). Here, for 
the continuance of individual life, particular or zanisms must 
be absent or not very numerous—beasts that browse, cater- 
pillars that devour leaves, aphides that suck juices. Among 
animals, however, the maintenance of the functional balance 
is both positively and negatively dependent on the amounts 
of surrounding organic agents. There must be an accessible 
sufficiency of the plants or animals serving for food; and of 
organisms that are predatory or parasitic or otherwise detri- 
mental, the number must not pass a certain limit. 

This dependence of the moving equilibrium in every indi- 
vidual organism on an adjustment of its forces to the forces 
of the environment, and the overthrow of this equilibrium 
by failure of the adjustment, is comprehensive of all cases. 
At first sight it does not seem to include what we call natural 
death; but only death by violence, or starvation, or cold, or 
drought. But in reality natural death, no less than every 
other kind of death, is caused by the failure to meet some 
outer action by a proportionate inner action. The apparent 
difference is due to the fact that in old age, when the 
quantity of force evolved in the organism gradually dimi- 
nishes, the momentum of the functions becomes step by step 
less, and the variations of the external forces relatively 
greater; until there finally comes an occasion when some 
quite moderate deviation from the average to which the 
feeble moving equilibrium is adjusted, produces in it a fatal 
perturbation. 


§ 817. The individuals of every species being thus depend- 


394 LAWS OF MULTIPLICATION. 


ent on certain environing actions ; and severally having their 
moving equilibria sooner or later overthrown by one or other 
of these environing actions; we have next to consider in 
what ways the environing actions are so met as to prevent 
extinction of the species. ‘There are two essentially different 
ways. There may be in each individual a small or great 
ability to adjust itself to variations of the agencies around 
it and to a small or great number of such varying agencies 
—there may be little or much power of preserving the 
balance of the functions. And there may be much or little 
power of producing new individuals to replace those whose 
moving equilibria have been overthrown. A few facts must 
be set down to enforce these abstract statements. 

There are both active and passive adaptations by which 
organisms are enabled to survive adverse influences. Plants 
show us but few active adaptations: that of the Pitcher-plant 
and those of the reproductive parts of some flowers (which do 
not, however, conduce to self-preservation) are exceptional 
instances. But plants have various passive adaptations; as 
thorns, stinging hairs, poisonous and acrid juices, repugnant 
odours, and the woolliness or toughness that makes their leaves 
uneatable. Animals exhibit far more numerous 
adjustments, both passive and active. In some cases they 
survive desiccation, they hybernate, they acquire thicker 
clothing, and so are fitted to bear unfavourable inorganic 
actions; and they are in many cases fitted passively to meet 
the adverse actions of other organisms, by bearing spines or 
armour or shells, by simulating neighbouring objects in colour 
or form or both, by emitting disagreeable odours, or by having 
disgusting tastes. In still more numerous ways they actively 
contend with unfavourable conditions. Against the seasons 
they guard by storing up food, by secreting themselves in 
crevices, or by forming burrows and nests. They save them- 
selves from enemies by developed powers of locomotion, taking 

he shape of swiftness or agility or aptitude for changing 
their media ; by their strength either alone or aided by wea- 


THE FACTORS. 395 


pons; lastly by their intelligence, without which, indeed, 
their other superiorities would avail them little. And then 
these various active powers serving for defence, become, in 
other cases, the powers that enable animals to aggress, and to 
preserve their lives by the success of their aggressions. 

The second process by which extinction is prevented—the 
formation of new individuals to replace the individuals 
destroyed—is carried on, as described in the chapter on 
“Genesis,” by two methods, the sexual and the asexual. 
Plants multiply by spontaneous fission, by gemmation, by 
proliferation, and by the evolution of young ones from de- 
tached cells and scales and leaves; and they also multiply 
by the casting off of spores and sporangia and seeds. In like 
manner among animals, there are varied kinds of agamo- 
genesis, from spontaneous fission up to parthenogenesis, all of 
them conducing to rapid increase of numbers; and we have 
the more familiar process of gamogenesis, also carried on 
in a great variety of ways. This formation of 
new individuals to replace the old, is, however, inadequately 
conceived if we contemplate only the number born or detached 
on each occasion. ‘There are four factors, all variable, on 
which the rate of multiplication depends. The first is the 
age at which reproduction commences; the second is the 
frequency with which broods are produced; the third is the 
number contained in each brood ; and the fourth is the length 
of time during which the bringing forth of broods con- 
tinues. There must be taken into account a further element 
—the amount of aid given by the parent to each germ in the 
shape of stored-up nutriment, continuous feeding, warmth, 
protection, &c.: on which amount of aid, varying between 
immensely wide limits, depends the number of the new indi- 
viduals that survive long enough to replace the old, and 
perform the same reproductive process. 

Thus, regarding every living organism as having a moving 
equilibrium dependent on environing forces, but ever liable 
to be overthrown by irregularities in those forces, and always 


396 LAWS OF MULTIPLICATION. 


so overthrown sooner or later; we see that each kind of 
organism can be maintained only by generation of new indi- 
viduals with a certain rapidity, and by helping them more 
or less fully to establish their moving equilibria. 


§ 318. Such are the factors with which we are here con- 
cerned. I have presented them in abstract shapes, for the 
purpose of showing how they are expressible in general terms 
of foree—how they stand related to the ultimate laws of re- 
distribution of matter and motion. | 

For the purposes of the argument now to follow, we may, 
however, conveniently deal with these factors under a more 
familiar guise. Ignoring their other aspects, we may class 
the actions which affect each race of organisms as forming 
two conflicting sets. On the one hand, by what we call 
natural death, by enemies, by lack of food, by atmospheric 
changes, &c., the race is constantly being destroyed. On the 
other hand, partly by the endurance, the strength, the swift- 
ness, and the sagacity of its members, and partly by their 
fertility, it 1s constantly being maintained. These conflicting 
sets of actions may be generalized as—the forces destructive 
of race and the forces preservative of race. So generalizing 
them, let us ask what are the necessary implications, 


CHAPTER It. 
A PRIORI PRINCIPLE. 


§ 319. The number of a species must at any time be either 
decreasing or stationary or increasing. If, generation after 
generation, its members die faster than others are born, the 
species must dwindle and finally disappear. If its rate of 
multiplication is equal to its rate of mortality, there can be 
no numerical change init. Andif the deductions by death 
are fewer than the additions by birth, the species must be- 
come more abundant. These we may safely set down as 
necessities. The forces destructive of race must be either 
greater than the forces preservative of race, or equal to them, 
or less than them; and there cannet but result these effects 
on number. 

We are here concerned only with races that continue to 
exist; and may therefore leave out of consideration those 
cases in which the destructive forces, remaining permanently 
in excess of the preservative forces, cause extinction. Prac- 
tically, too, we may exclude the stationary condition of a 
species ; for the chances are infinity to one against the main- 
tenance of a permanent equality between the births and the 
deaths. Hence, our inquiry resolves itself into this:—In 
races that continue to exist, what laws of numerical variation 
result from these variable conflicting forces, that are respec- 
tively destructive of race and preservative of race ? 


* § 820. Clearly if the forces destructive of race, when once 


398 LAWS OF MULTIPLICATION. 


in excess, had nothing to prevent them from remaining in 
excess, the race would disappear; and clearly if the forces 
preservative of race, when once in excess, had nothing to 
prevent them from remaining in excess, the race would go on 
increasing to infinity. In the absence of any compensating 
actions, the only possible avoidance of these opposite extremes 
would be an unstable equilibrium between the conflicting 
forces, resulting in a perfectly constant number of the species: 
a state which we know does not exist, and against the . 
existence of which the probabilities are, as already said, 
infinite. It follows, then, that as in every continuously- 
existing species, neither of the two conflicting sets of forces 
remains permanently in excess; there must be some way of 
stopping that excess of the one or the other which is ever 
occurring. 

How is this done? Should any one allege, in conformity 
with the old method of interpretation, that there is in each 
case a providential interposition to rectify the disturbed 
balance, he commits himself to the supposition that of the 
millions of species inhabiting the Earth, each one is yearly 
regulated in its degree of fertility by a miracle; since in no 
two years do the forces which foster, or the forces which 
check, each species, remain the same; and therefore, in no 
two years is there required the same fertility to balance 
the mortality. Few if any will say that God continually 
alters the reproductive activity of every parasitic fungus and 
every T'ape-worm or Trichina, so as to prevent its extinction 
or undue multiplication ; which they must say if they adopt 
the hypothesis of a supernatural adjustment. And in the 
absence of this hypothesis there remains only one other. 
The alternative possibility is, that the balance of the pre- 
servative and destructive forces is self-sustaining—is of the 
kind distinguished as a stable equilibrium: an equilibrium 
such that any excess of one of the forces at work, itself 
generates, by the deviation it produces, certain counter-forces 
that eventually out-balance it, and initiate an opposite devia- 


A PRIORI PRINCIPLE. 399 


tion. Let us consider how, in the case before us, such a stable 
equilibrium must be constituted. 


§ 321. When a season favourable to it, or a diminution of 
creatures detrimental to it, causes any species to become 
more numerous than usual; an immediate increase of certain 
destructive influences takes place. If it is a plant, the 
supposed greater abundance itself implies occupation of the 
available places for growth—an occupation which, leaving 
fewer such places as the multiplication goes on, itself becomes 
a check on further multiplication—itself causes a greater 
mortality of seeds that fail to root themselves. And after- 
wards, in addition to this passive resistance to continued 
increase, there comes an active resistance: the creatures that 
thrive at the expense of the species—the larvae, the birds, the 
herbivores—increase too. If it be an animal that has grown 
more numerous, then, unless by some exceptional coincidence 
a simultaneous and proportionate addition to the animals or 
plants serving for food has occurred, there must result a 
relative scarcity of food. .Enemies, too, be they beasts of 
prey or be they parasites, must quickly begin to multiply. 
Hence, each kind of organism, previously existing in some- 
thing like its normal number, cannot have its number raised 
without a rise of the destructive forces, negative and positive, 
quickly commencing. Both negative and posi- 
tive destructive forces must augment until this increase of 
the species is arrested. The competition for places on which 
to grow, if the species be vegetal, or for food if it be animal, 
must become more intense as the over-peopling of the habitat 
progresses; until there is reached the limit at which the 
mortality equals the reproduction. And as, at the same 
time, enemies will multiply with a rapidity which soon 
brings them abreast of the augmented supply of prey, the 
positive restraint they exert will help to bring about an 
earlier arrest of the expansion than pressure of population 
alone would cause. One more inference may be 


400 LAWS OF MULTIPLICATION. 


drawn. Had the species to meet no repressing influence. 
savé that negative one of relatively-diminished space or 
relatively-diminished food-supply, the cause leading to its 
increase might carry it up to the limit set by this, and there 
leave it: its enlarged number might be permanent. But 
the positive repressing Influence that has been called into 
existence, will prevent this. For the incrcase of enemies, 
commencing, as it must, after the increase of the species, 
and advancing in geometrical progression until it is itself 
checked in lke manner, will end in an excess of enemies. 
Whereupon must result a mortality of the species greater 
than its multiplication—a decrease which will continue until 
its habitat is underpeopled, its unduly-numerous enemies 
decimated by starvation, and the destroying agencies so 
reduced to a minimum. Whence will follow another in- 
crease. 

Thus, as before indicated (first Prin. § § 96, 133), there is 
here, as wherever antagonistic forces are in action, an alter- 
nate predominance of each, causing a rhythmical movement 
—a rhythmical movement which constitutes a moving equili- 
brium in those cases where the forces are not dissipated with 
appreciable rapidity, or are re-supplied as fast as they are 
dissipated. While, therefore, on the one hand, we see that 
the continued existence of a species necessarily implies some 
action by which the destructive and preservative forces are 
self-adjusted ; we see, on the other hand, that such an action 
is an inevitable consequence of the universal process of 
equilibration. 


-§ 822. Is this the sole equilibration that must exist? 
Clearly not. The temporary compensating adjustments of 
multiplication to mortality in each species, are but intro- 
ductory to the permanent compensating adjustments of mul- 
tiplication to mortality among species in general. The above 
reasoning would hold just as it now does, were all species 
equally prolific and all equally short-lived. It yields no 


A PRIORI PRINCIPLE. 401 


answer to the inquiries—why do their fertilities differ so 
enormously, or why do their mortalities differ so enormously P 
and how is the general fertility adapted to the general mor- 
tality ineach P The balancing process we have contemplated, 
can go on only within moderate limits—must fail entirely in 
the absence of a due proportion between the ordinary birth- 
rate and the ordinary death-rate. If the reproduction of 
mice proceeded as slowly as the reproduction of men, mice 
would be extinct before a new generation could arise: even 
did their natural lives extend to fifteen or sixteen years, it 
would -still be extremely improbable that any would for so 
long survive all the dangers they are exposed to. Con- 
versely, did oxen propagate as fast as infusoria, the race 
would die of starvation in a week. Hence, the minor adjust- 
ment of varying multiplication to varying mortality in each 
species, implies some major adjustment of average multipli- 
cation to average mortality. What must this adjustment be ? 

We have already seen that the forces preservative of race 
are two—ability in each member of the race to preserve 
itself, and ability to produce other members—power te main- 
tain individual life, and power to generate the species. 
These must vary inversely. When, from lowness of organi- 
zation, the ability to contend with external dangers is small, 
there must be great fertility to compensate for the conse- 
quent mortality ; otherwise the race must die out. When, 
on the contrary, high endowments give much capacity of 
self-preservation, a correspondingly-low degree of fertility is 
requisite. Given the dangers to be met as a constant quan- 
tity; then, as the ability of any species to meet them must 
be a constant quantity too, and as this is made up of the two 
factors—power to maintain individual life and power to mul- 
tiply—these cannot do other than vary inversely: one must 
decrease as the other increases. 

Tt needs but to conceive the results of nonconformity to 
this law, to see that every species must either conform to it 
or cease to exist. Suppose, first, a species whose individuals 

Vol. If 26 


402 LAWS OF MULTIPLICATION. 


having but small self-preservative powers are rapidly de- 
stroyed, to be at the same time without reproductive powers 
proportionately great. The defect of fertility, if extreme, 
will result in the death of one generation before another has 
grown up. If less extreme, it will entail a scarcity such 
that in the next generation sexual congress will be too infre- 
quent to maintain even the small number that remains; and 
the race will dwindle with increasing rapidity. If still less 
extreme, the consequent degree of rareness, while not so 
great as to prevent an adequate number of procreative 
unions, will be so great as to render special food very abundant 
and special enemies very few—will thus diminish the destruc- 
tive forces so much that the self-preservative forces will be- 
come relatively great: so great, relatively, that when com- 
bined with the small ability to propagate the species, they 
will suffice to balance the small destructive forces. Suppose, 
next, a species whose individuals have great powers of 
self-preservation, while they have powers of multiplication 
much beyond what is needful. The excess of fertility, if 
extreme, will cause sudden extinction of the species. by 
starvation. If less extreme, it must produce a permanent 
increase in the number of the species; and this, followed 
by intenser competition for food and augmented number 
of enemies, will involve such an increase of the dangers 
to individual life, that the great self-preserving powers of 
the individuals will not be more than sufficient to cope with 
them. That is to say, if the fertility is relatively too great, 
then the ability to maintain individual life inevitably becomes 
smaller, relatively to the requirements; and the inverse pro- 
portion is thus established. 

So that when, from comparing the different states of the 
same species, we go on to compare the states of different species, 
we see that there is an analogous adjustment—analogous 
in the sense that great mortality is associated with great 
multiplication, and small mortality with small multiplication. 
And we see that the unlikeness of ‘the cases consists merely 


A PRIORI PRINCIPLE. 403 


in this, that what is a temporary relation in the one is a per- 
manent relation in the other. 


§ 323. For the moment it does not concern us to inquire 
what is the origin of this permanent relation. That which 
we huve now to note, is simply that in some way or other 
there must be established an inverse proportion between the 
power to sustain individual life and the power to produce 
new individuals. Here it is enough for us to recognize this 
as a necessary truth. Whether or not the permanent rela- 
tion is self-adjusting in long periods of time, as the tempo- 
rary relation is self-adjusting in short periods of time, is a 
separate question. ‘The purpose of this chapter is fulfilled by 
showing that such a permanent relation must exist. 

But having recognized the @ priors principle that in races 
which continuously survive, the forces destructive of race 
must be equilibrated by the forces preservative of race; and 
that supposing these are constant, there must be an inverse 
proportion between self-preservation and race-preservation ; 
we may go on to inquire how this relation, necessary in 
theory, arises in fact. Leaving out the untenable hypothesis 
of a supernatural pre-adjustment, we have to ask in what 
way an adjustment comes about as a result of Hvolution. 
Ts it due to the survival of varieties in which the proportion 
of fertility to mortality happens to be the best? Or is the 
fertility adapted to the mortality in a more direct way? To 
these questions let us now address ourselves, 


CHAPTER III. 
OBVERSE A PRIORI PRINCIPLE, 


§ 324. When dealing with its phenomena inductively, we 
saw that however it may be carried on, Genesis ‘‘is a process 
of negative or positive disintegration ; and is thus essentially 
opposed to that process of integration, which is one element 
of individual evolution.” (§ 76.) Each new individual, whe- 
ther separated as a germ or in some more-developed form, is a 
deduction from the mass of a pre-existing individual or of two 
pre-existing individuals. Whatever nutritive matter is stored 
up along with the germ, if it be deposited in the shape of an 
ege, is so much nutritive matter lost to the parent. No 
drop of blood can be absorbed by the foetus, and no draught 
of milk sucked by the young when born, without taking 
from the mother tissue-forming and. force-evolving materials 
to an equivalent amount. And all subsequent supplies given 
to progeny, if they are nurtured, involve, to a parent or 
parents, so much waste in exertion that does not bring its 
return in assimilated food. 

Conversely, the continued aggregation of materials into one 
organism, renders impossible the formation of other organ- 
isms out of those materials. As much assimilated food as is 
united into a single whole, is so much assimilated food with- 
held from a plurality of wholes that might else have been 
produced. Given the absorbed nutriment as a constant 
quantity, and the longer the building of it up into a con- 


OBVERSE A PRIORI PRINCIPLE. 405 


crete shape goes on, the longer must be postponed any build- 
ing of it up into discrete shapes. And similarly, the larger 
the proportion of matter consumed in the functional actions 
of parents, the smaller must be the proportion of matter that 
can remain to establish and support the functional actions of 
offspring. | 

Though the necessity of these universal relations is toler- 
ably obvious as thus generally stated, it will be useful to dwell 
for a brief space on their leading aspects. 


§ 325. That disintegration which constitutes genesis, may 
be such as to disperse entirely the aggregate which integra- 
tion has previously produced—the parent may dissolve wholly 
into progeny. This dissolution of each aggregate into two 
or many aggregates, may occur at very short intervals, in 
which case the bulk attained can be but extremely small; or 
it may occur at longer intervals, in which case a larger bulk 
may. be attained. 

Instead of quickly losing its own individuality in the 
individualities of its offspring, each member-of the race may, 
after growing for a time, have portions of its substance begin 
to develop into the parental shape and presently detach 
themselves; and the parent, maintaining its own identity, 
may continue indefinitely so to produce young ones. But 
elearly, the earlier it commences doing this, and the more 
rapidly it does it, the sooner must the increase of its own 
bulk be stopped. | 

Or again, growth and development continuing for a long 
period without any deduction of materials, an individual of 
considerable size and organization may result; and then the 
abstraction of substance for the formation of new individuals, 
or rather the eggs of them, may be so great that as soon as 
the eggs are laid the parent dies of exhaustion—dies, that is, 
from an excessive loss of the nutritive matters needed for its 
own activities. 

Once more, the deduction of materials for the propagation 


406 LAWS OF MULTIPLICATION. 


of the species may be postponed long enough to allow of great 
bulk and complex structure being attained. The procreative 
subtraction then setting in, while it checks and presently 
stops growth, may be so moderate as to leave vital capital 
sufficient to carry on the activities of the parent; may go 
on as long as parental vigour suffices to furnish, without fatal 
result, the materials needed to produce young ones ; and may 
cease when such a surplus cannot be supplied, leaving the 
parental life to continue. 


§ 326. The opposite side of this antagonism has also 
several aspects. Progress of organic evolution may be shown 
in increased bulk, in increased structure, in increased amount 
or variety of action, or in combinations of these; and under 
any of its forms this carrying higher of each individuality, 
implies a correlative retardation in the establishment of new 
individualities. 

Other things equal, every addition to the bulk of an 
organism is an augmentation of its life. Besides being an 
advance in integration, it implies a greater total of acti- 
vities gone through in the assimilation of materials; and 
it implies, thereafter, a greater total of the vital changes 
taking place from moment to moment in all parts of the 
enlarged mass. Moreover, while increased size is thus, in so 
far, the expression of increased life, it is also, where the 
organism is active, the expression of increased ability to 
maintain life—increased strength. Aggregation of sub- 
stance is almost the only mode in which self-preserving 
power is shown among the lowest types; and even 
among the highest, sustaining the body in its integrity 
is that in which self-preservation fundamentally consists—is 
the end which the widest intelligence is indirectly made to 
subserve. While, on the one hand, the increase of tissue 
constituting growth is conservative both in essence and in 
result; on the other hand, decrease of tissue, either from 
injury, disease, or old age, is in both essence and result the 


OBVERSE A PRIORI PRINCIPLE. 407 


reverse. And if so, every addition to individual life thus 
implied, necessarily delays or diminishes the casting off of 
matter to form new individuals. 

Other things equal, too, a greater degree of organization 
involyes a smaller degree of that disorganization shown by 
the separation of reproductive gemmz and germs. Detach- 
ment of a living portion or portions from what was previously 
a living whole, is a ceasing of co-ordination ; and is therefore 
essentially at variance with that establishment of greater co- 
ordination which is achieved by structural development. In 
the extreme cases where a living mass is continually dividing 
and subdividing, it is manifest that there cannot arise much 
physiological division of labour; since progress towards 
mutual dependence of parts is prevented by the parts 
becoming independent. Contrariwise, it is equally clear 
that in proportion as the physiological division of labour is 
carried far, the separative process must be localized in some 
comparatively small portion of the organism, where it may 
go on without affecting the general structure—must become 
relatively subordinate. The advance that is shown by 
greater heterogeneity, must be a hindrance to multiplication 
in another way. For organization entails cost. That transfer 
and transformation of materials implied by differentiation, 
can be effected only by expenditure of force; and this sup- 
poses consumption of digested and absorbed food, which might 
otherwise have gone to make new organisms, or the germs of 
them. Hence, that individual evolution which consists in 
progressive differentiation, as well as that which consists in 
progressive integration, necessarily diminishes that species 
of dissolution, general or local, which propagation of the race 
exhibits. 

In active organisms we have yet a further opposition 
between self-maintenance and maintenance of the race. All 
motion, sensible and insensible, generated by an animal 
for the preservation of its life, is motion liberated from 
decomposed nutriment—nutriment which, if not thus decom- 


48 LAWS OF MULTIPLICATION. 


posed, would have been available for reproduction ; or rather 
-——might have been replaced by nutriment fitted for repro- 
ductive purposes, absorbed from other kinds of food. Hence, 
in proportion as the activities increase—in proportion as, by 
its more varied, complex, rapid, and vigorous actions, an 
animal gains power to support itself and to cope with sur- 
rounding dangers, it must lose power to propagate. 


§ 327. How may this antagonism be best expressed in a 
brief way? If self-preservation displayed itself in the 
highest organisms, as it does in the lowest, in little else but 
continuous growth ; and if race-preservation consisted always, 
as it does often, of nothing beyond detachment of portions 
from the parental mass; then the antagonism would be, 
throughout, the obviously-necessary one of integration and 
disintegration. Maintenance of the individual and propaga- 
tion. of the species, being respectively aggregative and separa- 
tive, it would be as self-evident that they vary inversely, as 
it is self-evident that addition and subtraction undo one 
another. But though the simplest types show us the opposi- 
tion of self-maintenance and race-maintenance almost wholly 
under this form; and though higher types, up to the most 
complex, exhibit it to a great extent under this form; yet, as 
we have just seen, this is not its only form. The total 
material monopolized by the individual and withheld from 
the race, must be stated as the quantity united to form its 
fabric, plus the quantity expended in differentiating its 
fabric, plus the quantity expended in its self-conserving 
actions. Similarly, the total material devoted to the race at 
the expense of the individual, includes that which is directly 
subtracted from the parent in the shape of egg or foetus, plus 
that which is directly subtracted in the shape of milk, plus 
that which is indirectly subtracted in the shape of matter 
consumed in the exertions of fostering the young. Hence 
this inverse variation is not expressible in simple terms of 
ageregation and separation. As we advance to more highly- 


OBVERSE A PRIORI PRINCIPLE. 409 


evolved organisms, the total cost of an individual becomes 
very much greater than is implied by the amount of tissue 
composing it. So, too, the total cost of producing each new 
individual becomes very much greater than that of its mere 
substance. And it is between these two total costs that the 
antagonism exists. 

We may, indeed, reduce the antagonism to a form compre- 
hensive of all cases, if we consider it as existing between the 
sums of the forces, latent and active, used for the two pur- 
poses. The molecules which make up a plant or animal, 
have been formed by the absorption of forces directly or 
indirectly derived from the sun; and hence the quantity of 
matter raised to the form called organic, which a plant or 
animal presents, is equivalent to a certain amount of force. 
Another amount of force is expressed by the totality of its 
differentiations. A further amount of force is that dissipated 
in its actions. And in these three amounts added together, 
we have the whole expense of the individual life. So, too, 
the whole expense of establishing each new individual 
includes—first the forces latent in the substance composing 
it when born or hatched; second the forces latent in the 
prepared nutriment afterwards supplied; and third the 
forces expended in feeding and protecting it. These two 
sets of forces being taken from a common. fund, it is manifest 
that either set can increase only by decrease of the other. 
If, of the force which the parent obtains from the environ- 
ment, much is consumed in its own life, little remains to be 
consumed in producing other lives; and, conversely, if there 
is a great consumption in producing other lives, it can only 
be where comparatively little is reserved for parental life. 

Hence, then, Individuation and Genesis are necessarily 
antagonistic. Grouping under the word Individuation all 
processes by which individual life is completed and main- 
tained; and enlarging the meaning of the word Genesis 
so as to include all processes aiding the formation and _per- 
fecting of new individuals; we see that the two are funda: 


410 LAWS OF MULTIPLICATION. 


mentally opposed. Assuming other things to remain the 
same—assuming that environing conditions as to climate, 
food, enemies, &c., continue constant; then, inevitably, every 
higher degree of individual evolution is followed by a lower 
degree of race-multiplication, and vice versd. Progress in 
bulk, complexity, or activity, involves retrogress in fertility ; 
and progress in fertility involves retrogress in bulk, com- 
plexity, or activity. 

This statement needs a slight qualification. For reasons 
to be hereafter assigned, the relation described is never com- 
pletely maintained ; and in the small departure from it, we 
shall find an admirable self-acting tendency to further the 
supremacy of the most-developed types. Here, however, this 
hint must suffice: explanation: would carry us too far out of 
our line of argument. For the present it will not lead us 
astray if we regard this inverse variation of Individuation 
and Genesis as exact. 


§ 328. Thus, then, the condition which each race must 
fulfil if it is to survive, is a condition which, in the nature of 
things, it ever tends to fulfil, In the last chapter we saw 
that a species cannot be maintained unless the power to 
preserve individual life and the power to propagate other 
individuals vary inversely. And here we have seen that, 
irrespective of an end to be subserved, these powers cannot 
do other than vary inversely. On the one hand, given a 
certain totality of destroying forces with which the species 
has to contend; and in proportion as its members have 
severally but small ability to resist these forces, it is requisite 
that they should have great ability to form new individuals, 
and vice versd. On the other hand, given the quantity of 
force, absorbed as food or otherwise, which the species can 
use to counterbalance these destroying forces; and in propor- 
tion as much of it 1s expended in preserving the individual, 
little of it can be reserved for producing new individuals, 
and vice versd. There is thus complete accordance between 


OBVERSE A PRIORI PRINCIPLE. 411 


the requirements considered under each aspect. The two 
necessities correspond. 

We might rest on these deductions and their several corol- 
laries. Without going further we might with safety assert 
the general truths that, other things equal, advancing evolu- 
tion must be accompanied by declining fertility ; and that, in 
the highest types, fertility must still further decrease if 
evolution still further increases. We might be sure that if, 
other things equal, the relations between an organism and its 
environment become so changed as permanently to diminish 
the difficulties of self-preservation, there will be a permanent 
increase in the rate of multiplication ; and, conversely, that a 
decrease of fertility will result where altered circumstances 
make self-preservation more laborious. 

But we need not content ourselves with these d priori 
inferences. If they are true, there must be an agreement 
between them and the observed facts. Let us see how far 
such an agreement is traceable. 


CHAPTER LY. 
DIFFICULTIES OF INDUCTIVE VERIFICATION. 


§ 329. Were all species subject to the same kinds and 
amounts of destructive forces, it would be easy, by comparing 
different species, to test the inverse variation of Individuation 
and Genesis. Or if either the power of self-preservation or 
the power of multiplication were constant, there would be 
little difficulty in seeing how the other changed as the 
destroying forces changed. But comparisons are nearly 
always partially vitiated by some want of parity. Hach 
factor, besides being variable as a whole, is compounded 
of factors that are severally variable. Not simply is the sum 
of the forces destructive of race different in every case; and 
not simply are both sets of forces preservative of race unlike in 
their totalities in every case; but each is made up of actions 
that bear such changing proportions to one another as to 
prevent any positive estimation of its amount. 

Before dealing with the facts as well as we can, it will be 
best to glance at the chief difficulties ; so that we may see the 
kind of verification which is alone possible. 


§ 330. Hither absolutely, or relatively to any species, 
every environment differs more or less from every other. 
There are the unlikenesses of media—air, water, earth, 
organic matter; severally involving special resistances to 
movement, and special losses of heat. There are the con 


DIFFICULTIES OF INDUCTIVE VERIFICATION. 413 


trasts of climate: here great expenditure for the maintenance 
of temperature is needed, and there very little; in one 
zone an organism is supplied with abundant light all the 
year round, and in another only for a few months; this 
region yields an almost unfailing supply of water, while that 
entails the exertion of travelling many miles every night for 
a draught. 

Permanent differences in the natures and distributions of 
aliment greatly interfere with the comparisons. The Swal- 
low goes through more exertion than the Sparrow in securing 
a given weight of food; but then their foods are dissimilar 
in nutritive qualities. There is a want of parallelism between 
the circumstances of those herbivores that live where the 
plains are annually covered for a time with rich herbage, 
but afterwards become parched up, and of those inhabiting 
more temperate regions. Insccts whose larvae feed on an 
abundant plant, as those of the genus Vanessu on the Nettle, 
have practically an environment very unlike that of insects 
such as Deilephila Euphorbie, whose larve feed on a com- 
paratively rare plant—the Sea-Spurge. 

Again, comparisons between creatures otherwise akin in 
their constitutions and circumstances, are hindered by ine- 
qualities in their relations to enemies. ‘Two animals, of 
which one is predatory and has no foes but parasites, while 
the other is much pursued, cannot properly be contrasted 
with a view to determining the influence of size or com- 
plexity. 

Without multiplying instances, it will be clear enough 
then that the aggregate of destructive actions, positive and 
negative, which each species has to contend with, is so 
undefinable in the amounts and kinds of its components, 
that nothing beyond a vague idea of its relative total can 
‘be formed. 


§ 331. Besides these immense variations in the outer 
actions to be counter-balanced, there are immense variations 


414 LAWS OF MULTIPLICATION. 


in the inner actions required to counter-balance them. Even 
if species were similarly conditioned, self-preservation would 
require of them extremely unlike expenditures of force. 

The cost of locomotion increases in a greater ratio than the 
size. In virtue of the law that the weights of animals increase 
as the cubes of their dimensions, while their strengths increase 
only as the squares of their dimensions (§ 46), a given speed 
requires a large animal to consume more substance in propor- 
tion to its weight, than it requires a small animal to consume ; 
and this law holding of all the mechanical actions, there 
results, other things equal, a difficulty of self-maintenance 
that augments in a more rapid ratio than the bulk. Nor 
must we overlook the further complication, that among 
aquatic creatures the variation of resistance of the medium 
partially neutralizes this effect. 

Again, the heat-consumption is a changing element in the 
{otal expense of self-preservation. Creatures that have tem- 
peratures scarcely above that of the air or water, may, other 
things equal, accumulate more surplus nutriment than 
creatures that have to keep their bodies warm spite of the 
continual loss by radiation and conduction. This difference 
of cost is modified by the presence or absence of natural 
clothing ; and it is also modified by unlikenesses of size. Here 
the bulky animals have the advantage: small masses cool- 
ing more rapidly than large ones. 

Dissimilarities of attack and defence are also causes of 
variation in the outlay for self-maintenance. A creature 
that has to hunt, as compared with another that gets a 
sufficiency of prey by lying in wait, or a creature that 
escapes by speed as compared with another that escapes by 
concealment, obviously leads a life that is physiologically 
more costly. Animals that protect themselves passively, 
as the Hedge-hog by its spines or as the Skunk and the 
Musk-rat by their intolerable odours, are relatively econo- 
mical; and have the more vital capital for other purposes. 

Amplification is needless. These instances will show that 


DIFFICULTIES OF INDUCTIVE VERIFICATION. 415 


anything beyond very general conceptions of the individual 
expenditures in different cases, cannot be reached. 


§ 332. Still more entangled are we among qualifying con- 
siderations when we contrast species in their powers of multi- 
plication. The total cost of Genesis admits of even less 
definite estimation than does the total cost of Individua- 
tion. I donot refer merely to the truth that the degree of 
fertility depends on four factors—the age of commencing 
reproduction, the number in each brood, the frequency of the 
broods, and the time during which broods continue to be 
repeated. There are many further obstacles in the way of 
comparisons. 

Were all multiplication carried on sexually, the problem 
would be less involved; but there are many kinds of asexual 
multiplication alternating with the sexual. This asexual 

multiplication is in some cases perpetual instead of occa- 
sional; and often has more forms than one in the same 
species. The result is that we have to compare what is here 
a periodic process with what is elsewhere a cyclical process 
partly continuous and partly periodic—the calculation of fer- 
tility in this last case being next to impossible. 

We have to avoid being misled by the assumption that the 
cost of Genesis is measured by the number of young produced, 
instead of being measured, as it is, by the weight of nutri- 
ment abstracted to form the young, plus the weight con- 
sumed in caring for them. This total weight may be 
very diversely apportioned. In contrast to the Cod with its 
million of small ova spawned without protection, we may 
put the Hippocampus or the Pipe-fish, with its few relatively- 
large ova carried about by the male in a caudal pouch, or 
seated in hemispherical pits in its skin; or we may put the 
still more remarkable genus Arius, and especially Arius 
Boakeii—a fish some six or seven inches long, which produces 
ten or a dozen eggs as large as marbles, that are carried by 
the male in his mouth till they are hatched. Here though 


416 LAWS OF MULTIPLICATION. 


the degrees of fertility, if measured by the numbers of 
fertilized germs deposited, are extremely unlike, they are 
less unlike if measured by the numbers of young that are 
hatched and survive long enough to take care of themselves ; 
nor will the tax on the parent-Cod seem so immensely dif- 
ferent from that on the parent-Avius, if the masses of the ova, 
instead of their numbers, are compared. Again, 
while sometimes the parental loss ts little else but the matter 
deducted to form eggs, &c.; at other times it takes the 
shape of a small direct deduction joined with a large indirect 
outlay. The Mason-wasp furnishes a typical instance. In 
journeyings hither and thither to fetch bit by bit the 
materials for building a cell; in putting together these 
materials, as well as in secreting glutinous matter to act as 
cement; and then, afterwards, in the labour of seeking for, 
and carrying, the small caterpillars with which it fills up the 
cell to serve its larva with food when it emerges from the 
egea; the Mason-wasp probably expends more substance than 
is contained in the egg itself. And this supplementary ex- 
penditure is manifestly so great, that but few eggs can be 
housed and provisioned. . 
Estimates of the cost of Genesis are further complicated by 
variations in the ratio borne by the two sexes. Among 
Fishes the mass of milt approaches in size the mass of spawn; 
but amorg higher Vertebrata the substance lost by the one 
sex in the shape of sperm-cells is small compared with that 
lost 'by the other sex in the shape of albumen stored-up in 
the eggs, or blood supplied to the foetus, or milk given to the 
young. Then there come the differences of indirect tax 
on males and females. ‘While, frequently, the fostering of 
the young devolves entirely on the female, occasionally, the 
male undertakes it wholly or in part. After building a 
nest, the male Stickleback guards the eggs till they are 
hatched ; as does also the great Silurus glanis for some forty 
days, during which he takes no food. And then, among most 
birds, we'have the male occupied in feeding the female during 


DIFFICULTIES OF INDUCTIVE VERIFICATION. 417 


incubation, and the young afterwards. Evidently all these 
differences affect the proportion between the total cost of re- 
production and the total cost of individuation. 

Whether the species is monogamous or polygamous, and 
whether there are marked differences of size or of structure 
between males and females, are also questions not to be over- 
looked. If there are many females to one male, the total 
quantity of assimilated matter devoted by each generation to 
the production of a new generation, is greater than if there 
is a male to each female. Similarly, where the requirements 
are such that small males will suffice, the larger quantity of 
food left for the females, makes possible a greater surplus 
available for reproduction. And where, as in some of the 
Cirrhipedia, or such a parasite as Spherularia Bombi, the 
female is a thousand or many thousand times the size of the 
male, the reproductive capacity is almost doubled: the effect 
on the rate of multiplication being something like that which 
would result if any ordinary race could have all its males 
replaced by fertile females. Conversely, where the 
habits of the race render it needless that both sexes should 
have developed powers of locomotion—where, as in the Glow- 
worm and sundry Lepidoptera, the female is wingless while 
the male has wings—the cost of Individuation not being so 
great for the species as a whole, there arises a greater reserve 
for Genesis: the matter which would otherwise have gone to 
the production of wings and the using of them, may go to 
the production of ova. | 

Other complications, as those which we see in Bees and 
Ants, might be dwelt on; but the foregoing will amply serve 
the intended purpose. 


§ 333. To ascertain by comparison of cases whether Indi- 
viduation and Genesis vary inversely, is thus an under- 
taking so beset with difficulties, that we might despair of any 
satisfactory results, were not the relation too marked a oné 


to be hidden even by all these complexities. Species are 
VOL. IL. 27 


418 LAWS OF MULTIPLICATION. 


so extremely contrasted in their degrees of evolution, and so 
extremely contrasted in their rates of multiplication, that the 
law of relation between these characters becomes unmis- 
takable when the evidence is looked at in its ensemble. This 
we shall soon find on ranging in order a number of typical 
cases. . 

In doing this it will be convenient to neglect, temporarily, 
all unlikenesses among the circumstances in which organ- 
isms are placed. At the outset, we will turn our attention 
wholly to the antagonism displayed between the integrative 
process which results in individual evolution and the disinte- 
grative process which results in multiplication of individuals ; 
and this we will consider first as we see it under the several 
forms of agamogenesis, and then as we see it under the seve- 
ral forms of gamogenesis. We will next look at the anta- 
gonism between propagation and that evolution which is 
shown by increased complexity. And then we will consider 
the remaining phase of the antagonism, as it exists between 
the degree of fertility and the degree of evolution expressed 
by activity. 

Afterwards, passing to tlie varying relations between 
organisms and their environments, we will note how relative 
increase in the supply of food, or relative decrease in the 
quantity of force expended by the individual, entails relative 
increase in the quantity of force devoted to multiplication, 
and wice versd. 

Certain minor qualifications, together with sundry impor- 
tant corollaries. may then be entered upon. 


CHAPTER V. 
ANTAGONISM BETWEEN GROWTH AND ASEXUAL GENESIS. 


§ 334. When illustrating, in Part IV., the morphological 
composition of plants and animals, there were set down in 
groups, numerous facts which we have here to look at from 
another point of view. Then we saw how, by union of small 
simple aggregates, there are produced large compound aggre- 
gates. Now we have to observe the reactive effect of this 
process on the relative numbers of the aggregates. Our 
present subject is the antagonism of Individuation and 
Genesis as seen under its simplest form, in the self-evident 
truth that the same quantity of matter may be divided into 
many small wholes or few large wholes; but that number 
negatives largeness and largeness negatives number. 

In setting down some examples, we may conveniently 
adopt the same arrangement as before. We will look at the 
facts as they are presented by vegetal aggregates of the first 
order, of the second order, and of the third order; and then 
as they are presented by animal aggregates of the same three 


orders. 


§ 335. The ordinary unicellular plants are at once micro- 
scopic and enormously prolific. The often cited Protococcus 
nivalis, which shows its immense powers of multiplication by 
reddening wide tracts of snow in a single night, does this by 
developing in its cavity a brood of young cells, which, being 


ys 


420) LAWS OF MULTIPLICATION. 


presently set free by the bursting of the parent-cell, severally 
grow and quickly repeat the process. The like occurs among 
sundry of those kindred forms of minute Alge which, by 
their enormous numbers, sometimes suddenly change pools to 
an opaque green. So, too, the Desmidiacee often multiply so 
greatly as to colour the water; and among the Diatomacee 
the rate of genesis by self-division, “is something really extra- 
ordinary. So soon as a frustule is divided into two, each of 
the latter at once proceeds with the act of self-division ; so 
that, to use Professor Smith’s approximative calculation of 
the possible rapidity of multiplication, supposing the process 
to occupy, in any single instance, twenty-four hours, ‘ we 
should have, as the progeny of a single frustule, the amazing 
number of one thousand millions in a single month.’” In 
these cases the multiplication is so carried on that the parent 
is lost in the offspring—the old individuality disappears 
either in the swarms of zoospores it dissolves into, or in the 
two or four new individualities simultaneously produced by 
fission. Vegetal aggregates of the first order, 
have, however, a form of agamogenesis in which the parent 
individuality is not lost: the young cells arise from the old 
cells by external gemmation. This process, too, repeated as 
it is at short intervals, results in immense fertility. The 
Yeast-fungus, which in a few hours thus propagates itself 
throughout a large mass of wort, offers a familiar example. 
In certain compound forms that must be classed as plants 
of the second order of aggregation, though very minute ones, 
self-division similarly increases the numbers at high rates. 
The Sarcina ventriculi, a parasitic plant that infests the 
stomach and swarms afresh as fast as previous swarms are 
vomited, shows us a spontaneous fission of clusters of cells. 
An allied mode of increase occurs in Gonium pectorale: each 
cell of the cluster resolving itself into a secondary cluster, 
and the secondary clusters then separating. ‘“ Supposing, 
which is very probable, that a young Gonium after twenty- 
four hours is capable of development by fission, it follows 


GROWTH AND ASEXUAL GENESIS. 421 


that under favourable conditions a single colony may on the 
second day develop 16, on the third 256, on the fourth 4,096, 
and at the end of a week 268,435,456 other organisms like 
itself” In the Volvocine this continual dissolution of a primary 
compound individual into secondary compound individuals, 1s 
carried on endogenously—the parent bursting to liberate the 
young ; and the numbers arising by this method, also are some- 


times so great as to tint large bodies of water. More 
fully established and organized aggregates of the second 


order, such as the higher Thallogens and the lower Acrogens, 
do not sacrifice their individualities by fission; but never- 
theless, by the kindred process of gemmation, are continually 
hindered in the increase of their individualities. The gemmee 
called tetraspores are cast off in great numbers by the marine 
Alge. Among those simple Jungermanniacee which consist 
of single fronds, the young ones that bud out grow for a time 
in connexion with their parents, send rootlets from their 
under sides into the soil, and presently separate themselves— 
a habit which augments the number of individuals in propor- 
tion as it checks their growths. 

Plants of the third order of composition, arising by arrest 
of this separation, exhibit a further corresponding decrease 
in the abundance of the aggregates formed. Acrogens of 
inferior types, in which the axes produced by integration of 
fronds are but small and feeble, are characterized by the 
habit of throwing off bulbils—bud-shaped axes which, falling 
and taking root, add to the number of distinct individuals. 
This agamic multiplication, very general among the Mosses 
and their kindred, and not uncommon under a modified 
form in such higher types as the Ferns, many of which 
produce young ones from the surfaces of their fronds, becomes 
very unusual among Phenogams. The detachment of bulbils, 
though not unknown among them, is exceptional. And while 
it is true that some flowering plants, as the Strawberry, 
multiply by a process allied to gemmation, yet this is 
anything but characteristic of the class. A leading trait of 


422 LAWS OF MULTIPLICATION. 


these highest groups, to which the largest members of the 
vegetal kingdom belong, is that agamogenesis has so far 
ceased that it does not originate independent plants. Though 
the axes which, budding one out of another, compose a tree, 
are the equivalents of asexually-produced individuals; yet 
the asexual production of them stops short of separation. 
These vast integrations arise where spontaneous disintegra- 
tion, and the multiplication effected by it, have come to an 
end. 

Thus, not forgetting that certain Phznogams, as Begonta 
phyllomaniaca, revert to quite primitive modes of increase, we 
may hold it as beyond question that while among the most 
minute plants asexual multiplication is universal, and pro- 
duces enormous numbers in short periods, it becomes step by 
step more restricted in range and frequency as we advance to 
large and compound plants; and disappears so generally 
from the largest, that its occurrence is regarded as anomalous. 


§ 336. Parallel examples showing the inverse variation of 
erowth and asexual genesis among animals, make clear the 
purely quantitative nature of this relation under its original 
form. Of the Amba it is said that ‘‘ when a large variable 
process has been shot out far from the chief mass and become 
enlarged at the extremity, the expanded end retains its posi- 
tion, whilst the portion connecting it with the body becomes 
finer and finer by being withdrawn into the parent mass, 
until it at last breaks across, leaving a detached piece, which 
immediately on its own account shoots out processes, and 
manifests an independent existence. ‘This phenomenon is 
therefore one of simple detachment, and cannot rightly be 
called a process of fission.” But it shows us, nevertheless, 
how the primordial form of multiplication is nothing more 
than a separation, instead of a continued union, of the grow- 
ing mass. Among the Protosca, as among the — 
Protophyta, there occurs that process by which the in- 
dividuality of the parent is wholly lost in producing offspring 


GROWTH AND ASEXUAL GENESIS. 423 


—the breaking up of the parental mass into a number of 
germs. An*example is supplied by one of the lowest of the 
class—the Gregarina. This creature, which is nothing more 
than a minute spheroidal nucleated mass of protoplasm, 
having a structureless outer layer denser than the rest, but 
being without mouth or any organ, resolves itself into a 
multitude of still more minute masses, which when set free 
by bursting of the envelope, shortly become Ameba-form, 
and severally assuming the structure of the parent, go 
through the same course. Some of the Jnfusoria, as for in- 
stance those of the genus [o/poda, similarly become encysted 
and subsequently break up into young ones. The 
more familiar mode of increase among these animal-aggre- 
gates of the first order, by fission, though it sacrifices the 
parent individuality by merging it in the individualities of 
the two produced, sacrifices it less completely than does the 
dissolution into a great number of germs. Occurring, how- 
ever, as this fission does, very frequently, and being com- 
pleted, in some cases that have been observed, in the course 
of half-an-hour, it results in immensely-rapid multiplication. 
If all its offspring survive, and continue dividing them- 
selves, a single Paramecium is said to be capable of thus 
originating 268 millions in the course of a month. Nor is 
this the greatest known rate of increase. Another animalcule, 
visible only under a high magnifying power, “ is calculated 
to generate 170 billions in four days.” And these enormous 
powers ofspropagation are accompanied by a minuteness so 
extreme, that of some species one drop of water would contain 
as many individuals as there are human beings on the Earth! 
Making allowauce for exageeration in these estimates, it is 
beyond question that among these smallest of animals the 
rate of asexual multiplication is by far the greatest; and 
this suffices for the purposes of the argument.* 


* That these estimated rates are not greater than is probable, may be 
inferred from such observations as that of Mr. Brightwell on the buds 
of Zoothamnium. ‘‘At nine in the morning, one of these buds, or ova, was 


424 LAWS OF MULTIPLICATION. 


Of animal aggregates belonging to the second order, that 
multiply asexually with rapidity, the familiar Polypes 
furnish conspicuous examples. By gemmation in most 
cases, in other cases by fission, and in some cases by both, 
the agamogenesis is carried on among these tribes. As 
shown in Fig. 148, the budding of young ones from the 
parent Hydra is carried on so actively, that before the oldest 
of them is cast off half-a-dozen or more others have reached 
various stages of growth; and even while still attached, the 
first-formed of the group have commenced budding out 
from their sides a second generation of young ones. In the 
Hydra tuba this gemmiparous multiplication is from time to 
time interrupted by a transverse splitting-up of the body into 
segments, which successively separate and swim away: the 
result of the two processes being, that in the course of a 
season there are produced from a single germ, great numbers 
of young Meduse, which are the adult or sexual forms of the 
species. Respecting Cclenterate animals of this degree of 
composition, it may be added that when we ascend to the 
larger kinds we find asexual genesis far less active. 
Though comparisons are interfered with by differences of 
structure and mode of life, yet the contrasts are too striking 
to have their meanings much obscured. If, for instance, we 
take a solitary -Actinozoon and a solitary Hydrozoon, we see 
that the relatively-great bulk of the first, goes along with a 
relatively-slow agamogenesis. The common Sea-anemones 
are but occasionally observed to undergo self-division. their 
numbers are not rapidly increased by this process. A 
higher class. of secondary aggregates exemplifies the same 


observed fixed to the glass by a sheathed pedicle; a ciliary motion, became 
perceptible at the top of the bulb; and at ten it had divided longitudinally 
into two buds, each supported by a short stalk. The ciliary motion continued 
in the centre of each of these two buds, which by degrees expanded longitudi- 
nally, and at twelve had become four buds. By four in the afternoon, these 
four buds had divided in like manner and increased to nine, with an elongated 
footstalk, and interior contractile muscle.” 


GROWTH AND ASEXUAL GENESIS. 425 


general truth with a difference. In the smaller members the 
agamogenesis is incomplete, and in the larger it disappears. 
Each sub-section of the Mod/uscoida shows us this. The gemma- 
tion of the minute Po/yzoa, though it does not end in the sepa- 
ration of the young individuals, habitually goes to the extent 
of producing families of partially-independent individuals ; 
but their near allies the Brachiopoda, which immensely exceed 
them in size, are solitary and not gemmiparous. So, too, is 
it with the Ascidioida. And then among the true Mollusca, 
including all the largest forms belonging to this sub-kingdom, 
no such thing is known as fission or gemmation. : 

Take next the Annulosa, including under this title the 
Annuloida. When treating of morphological composition, 
reasons were given for the belief that the annulose animal is 
an aggregate of the third order, the segments of which, 
produced one from another by gemmation, originally 
became separate, as they still become in the cestoid 
Entozoa; but that by progressive integration, or arrested 
disintegration, there resulted a type in which many such 
segments were permanently united (§§ 205-7). Part of the 
evidence there assigned, is evidence to be here repeated. in 
illustration of the direct antagonism of Growth and Asexual- 
Genesis. We saw how, among the lower Annelids, the string 
of segments produced by gemmation presently divides trans- 
versely into two strings; and how, in some cases, this resolu- 
tion of the elongating string of segments into groups that 
are to form separate individuals, goes on so actively that as 
many as six groups are found in different stages of progress 
to ultimate independence—a fact implying a high rate of 
fissiparous multiplication. Then we saw that, in the superior 
annulose types, distinguished in the mass by including the 
larger species, fission does not occur. ‘The higher Annelids 
do not propagate in this way; there is no known case of new 
individuals being so formed among the JJvyriapoda; nor do 
the Crustaceans afford us a single instance of this primordial 
mode of increase. It is, indeed, true that while 


426 LAWS OF MULTIPLICATION. 


articulate animals never multiply asexually after this simplest 
method, and while they are characterized in the mass by the 
cessation of agamogenesis of every kind, there nevertheless 
occur in a few of their small species, those higher forms of 
agamogenesis known as parthenogenesis, pseudo-partheno- 
genesis and internal metagenesis; and that by these some of 
them multiply very rapidly. Hereafter we shall find, in the 
interpretation of these anomalies, further support for the 
general doctrine. 

To the above evidence has to be added that which the 
‘Vertebrata present. This may be very briefly summed up. 
On the one hand, this class, whether looked at in the aggre- 
gate or in its particular species, immensely exceeds all other 
classes in the sizes of its individuals; and on the other hand, 
agamogenesis under any form is absolutely unknown in it. 


§ 337. Such are a few leading facts serving to show how 
deduction is inductively verified, in so far as the anta- 
gonism between Growth and Asexual Genesis is con- 
cerned. In whatever way we explain this opposition of 
the integrative and disintegrative processes, the facts and 
their implications remain the.same. Indeed we need not 
commit ourselves to any hypothesis respecting the physical 
causation: it suffices to recognize the results under their 
most general aspects. We cannot help admitting there are 
at work these two antagonist tendencies to aggregation and 
separation; and we cannot help admitting that the propor- 
tion between the aggregative and separative tendencies, must 
in each case determine the relation between the increase in 
bulk of the individual and the increase of the race in number. 

The antithesis is as manifest @ posteriori as it is neces- 
sary ad priori. While the minutest organisms multiply 
asexually in their millions; while tke small compound 
types next above them thus multiply in their thousands ; 
while larger and more compound types thus multiply in their 
hundreds and their tens; the largest types do not thus 


GROWTH AND ASEXUAL GENESIS. 427 


multiply at all. Conversely, those which do not multiply 
asexually at all, are a billion or a million times the size of 
those which thus multiply with greatest rapidity ; and are a 
thousand times, or a hundred times, or ten times the size of 
those which thus multiply with less and less rapidity. With- 
out saying that this inverse proportion is regular, which, as we 
shall hereafter see, it cannot be, we may unhesitatingly assert 
its average truth. That the smallest organisms habitually 
reproduce asexually with immense rapidity ; that the largest 
organisms never reproduce at all in this manner; and that 
between these extremes there is a general decrease of asexual 
reproduction along with an increase of bulk; are proposi- 
tions that admit of no dispute. 


CHAPTER VI. 
ANTAGONISM BETWEEN GROWTH AND SEXUAL GENESIS. 


§ 338. In so far as it is a process of separation, sexual 
genesis is like asexual genesis; and is therefore, equally with 
asexual genesis, opposed to that aggregation which results in 
growth. Whether a deduction is made from one parent or 
from two, whether it is made from any part of the body 
indifferently or from a specialized part, or whether it is made 
directly or indirectly, it remains in any case a deduction; 
and in proportion as it is great, or frequent, or both, it must 
restrain the increase of the individual. 

Here we have to group tegether the leading illustrations 
of this truth. We will take them in the same order as 
before. 


§ 339. The lowest vegetal forms, or rather, we may say, 
those forms which we cannot class as either distinctly vegetal 
or distinctly animal, show us a process of sexual multiplica- 
tion that differs much less from the asexual process than in 
the higher forms. The common character which distinguishes 
sexual from asexual genesis, is that the mass of protoplasm 
whence a new generation is to arise, has been produced by the 
union of two portions of matter that were before more widely 
separated. I use this general expression, because among the 
simplest Algae, this 1s not invariably matter supplied by 
different individuals: certain Diatomacee exhibit within a 
single cell, the formation of a sporangium by a drawing 


GROWTH AND SEXUAL GENESIS. 429 


together of the opposite halves of the endochrome into a 
ball. Mostly, however, sporangia are products of conjuga- 
tion. The endochromes of two cells unite to form the germ- 
mass; and these conjugating cells may be either entirely 
independent, as in many Desmidiacee and in the Palmelle ; or 
they may be two of the adjacent cells forming a thread, as in 
some Conjugatee ; or they may be cells belonging to adjacent 
threads, as in Zygnema. But whether it is originated by a 
single parent-cell, or by two parent-cells, the sporangium, 
after remaining quiescent until there recur the fit conditions 
for growth, breaks up into a multitude of spores, each of which 
produces an individual that multiplies asexually ; and the fact 
here to be noted is, that as the entire contents of the parent- 
cells unite to form the sporangium, their individualities are lost 
in the germs of a new generation. In these minute simple 
types, sexual propagation just as completely sacrifices the life 
of the parent or parents, as does that form of asexual propa- 
gation in which the endochrome resolves itself directly into 
zoospores. And in the one case as in the other, this sacrifice 
is the concomitant of a prodigious fertility. Slightly 
in advance of this, but still showing us an almost equal loss 
of parental life in the lives of offspring, is the process seen in 
such unicellular Alge as Hydrogastrum, and in minute Fungi 
of the same degree of composition. These exhibit a relatively - 
enormous development of the spore-producing part, and an 
almost entire absorption of the parental substance into it. 
As evidence of the resulting powers of multiplication, we 
have but to remember that the spread of mould over stale 
food, the rapid destruction of crops by mildew, and other 
kindred oecurrences, are made possible by the incalculably 
numerous spores thus generated and universally dispersed. 
Plants a degree higher in composition, supply a parallel 
series of illustrations. We have among the larger Fungi, in 
which the reproductive apparatus is relatively so enormous as 
to constitute the ostensible plant, a similar subordination of 
the individual to the race, and a similarly-immense fertility. 


430 LAWS OF MULTIPLICATION. 


Thus, as quoted by Dr. Carpenter, Fries says—“in a single 
individual of Reticularia maxima, I have counted (calculated ?) 
10,000,000 sporules.” It needs but to note the clouds of 
particles, so minute as to look like smoke, which ripe puff- 
balls give off when they are burst, and then to remember 
that each particle is a potential fungus, to be impressed with 
the almost inconceivable powers of propagation which these 
plants possess. The Lichens, too, furnish examples. 
Though they are nothing like so prolific as the Fungi (the 
difference yielding, as we shall hereafter see, further support 
to the general argument), yet there is a great production of 
germs, and a proportionate sacrifice of the parental indi- 
viduality. Considerable areas of the frond here and there 
develop into apothecia and spermagonia, which resolve them- 
selves into sperm-cells and germ-cells. Some con- 
trasts presented by the higher 4/ge may also be named as 
exemplifying the inverse proportion between the size of the 
individual and the extent of the generative structures. While 
in the smaller kinds relatively large portions of the fronds are 
transformed into reproductive elements, in the larger kinds 
these portions are relatively small: instance the Macrocystis 
pyrifera, a gigantic sea-weed, which sometimes attains a 
length of 1,500 feet, of which Dr. Carpenter remarks— 
“This development of the nutritive surface takes place at 
the expense of the fructifying apparatus, which is here quite 
subordinate.” 

When we turn to vegetal aggregates of the third order of 
composition, facts having the same meaning are conspicuous. 
On the average these higher plants are far larger than 
plants of a lower degree of composition; and on the average 
' their rates of sexual reproduction are far less. Similarly if, 
among Acrogens, Endogens, and Exogens, we compare the 
smaller types with the larger, we find them proportionately 
more prolific. This is not manifest if we simply calculate 
the number of seeds ripened by an individual in a single 
season; but it becomes manifest if we take into account the 


GROWTH AND SEXUAL GENESIS. 431 


further factor which here complicates the result—the age at 
which sexual genesis commences. The smaller Pheenogams 
are mostly either annuals, or perennials that die down 
annually; and seeding as they do annually before their 
deaths, or the deaths of their reproductive parts, it results 
that in the course of a year, each gives origin to a multitude 
of potential plants, of which every one may the next year, if 
preserved, give origin to an equal multitude. Supposing but 
a hundred offspring to be produced the first year, ten 
thousand may be produced in the second year, a million in 
the third, a hundred millions in the fourth. Meanwhile, 
what has been the possible multiplication of a large Pha- 
nogam? While its small congener has been seeding and 
dying, and leaving multitudinous progeny to seed and die, it 
has simply been growing; and may so continue to grow for 
ten or a dozen years without bearing fruit. Before a Cocoa- 
nut tree has ripened its first cluster of nuts, the descendants 
of a wheat plant, supposing them all to survive and multiply, 
will have become numerous enough to occupy the whole 
surface of the Earth. So that though, when it begins to 
bear, a tree may annually shed as many seeds as a herb, yet 
in consequence of this delay in bearing, its fertility is incom- 
parably less; and its relatively-small fertility becomes still 
further reduced where, as in Lodoicea Sechellarum, the seeds 
take two years from the date of fertilization to the date of 
germination. 


§ 340. Some observers state that in certain Protozoa there 
occurs a process of conjugation akin to that which the 
Protophyta exhibit—a coalescence of the substance of two 
individuals to form a germ-mass. This has been alleged 
' more especially of <Actinophrys. The statement is question- 
able; but if proved true, then of the minute forms that 
appear to be more animal than vegetal in their characters, 
some have a mode of sexual multiplication by which the 
parents are sacrificed bodily in the production of a new 


432 LAWS OF MULTIPLICATION. 


generation. A modified mode, apparently not fatal to the 
parents, has been observed in certain of the more developed 
Infusoria. Our knowledge of these microscopic types is, 
however, so rudimentary that evidence derived from them 
must be taken with a qualification. 

Among small animal aggregates of the second order, the 
first to be considered are of course the Celenterata. A Hydra 
occasionally devotes a large part of its substance to sexual 
genesis. In the walls of its body groups of ova, or sperma- 
tozoa, or both, take their rise; and develop into masses 
greatly distorting the creature’s form, and leaving it greatly 
diminished when they escape. Here, however, gamogenesis is 
obviously supplementary to agamogenesis—the immensely 
rapid multiplication by budding continues as long as food is 
abundant and warmth sufficient, and is replaced by gamo- 
genesis only at the close of the season. A. better 
example of the relation between small size and active gamo- 
genesis is supplied by the Planaria, which does not multiply 
asexually with so much rapidity. ‘The generative system is 
here enormous. Ova are developed all through the body, 
occupying everywhere the interspaces of the assimilative 
system; so that the animal may be said to consist of a part 
that absorbs nutriment and a part that transforms that nutri- 
ment into sperm-cells and germ-cells. Even saying nothing 
of the probably-early maturity of these animals, and there- 
fore frequent repetition of sexual multiplication, it is clear 
that their fertility must be very great. 

The Annwlosa, including among them the inferior kindred 
types, have habits and conditions of life so various that only 
the broadest contrasts can be instanced in support of the pro- 
position before us. Of the microscopic forms belonging to 
this sub-kingdom, the Jotifera may be named as having, 
along with small bulk, a great rate of sexual increase. Hyda- 
tina senta “is capable of a four-fold propagation every twenty- 
four or thirty-hours, bringing forth in this time four ova, 
which grow from the embryo to maturity, and exclude their 


GROWTH AND SEXUAL GENESIS. 433 


fertile ova in the same period. The same individual, pro- 
ducing in ten days forty eggs, developed with the rapidity 
above cited, this rate, raised to the tenth power, gives one 
million of individuals from one parent, on the eleventh day 
four millions, and on the twelfth day sixteen millions, and so 
on.” Ascending from this extreme, the differences 
of organization and activity greatly complicate the inverse 
variation of fertility and bulk. Bearing in mind, how- 
ever, that the rate of multiplication depends much less on the 
number of each brood than on the quickness with which 
maturity is reached and a new generation commenced, it will 
be obvious that though Annelids produce great numbers of 
ova, yet as they do this at comparatively long intervals, their 
rates of increase fall immensely below that just instanced in 
the Rotifers. And when at the other extreme we come to 
the large articulate animals, such as the Crab and the Lobster, 
the further diminution of fertility is seen in the still longer 
delay that occurs before each new generation begins to re- 
produce. 

Perhaps the best examples are supplied by vertebrate 
animals, and especially those that are most familiar to us. 
Comparisons between Fishes are unsatisfactory, because of 
our ignorance of their histories. In some cases Fishes equal 
in bulk produce widely different numbers of eggs; as the 
Cod which spawns a million at once, and the Salmon by 
which nothing like so great a number is spawned. But then 
the eggs are very unlike in size; and if the ovaria of the two 
fishes be compared, the difference between their masses 1s 
comparatively moderate. There are, indeed, contrasts which 
seem at variance with the alleged relation; as that between 
the Cod and the Stickleback, which, though so much smaller, 
produces fewer ova. The Stickleback’s ova, however, are 
relatively large; and their total bulk bears as great a ratio to 
the bulk of the Stickleback as does the bulk of the Cod’s ova 
to that of the Cod. Moreover, if, as is not improbable, the 


reproductive age is arrived at earlier by the Stickleback than 
VoL. I. 28 


4384 LAWS OF MULTIPLICATION. 


by the Cod, the fertility of the species may be greater not- 
withstanding the smaller number produced by each indi- 
vidual. Hividence that admits of being tolerably 
well disentangled is furnished by Birds. They differ but 
little in their grades of organization; and the habits of life 
throughout extensive groups of them are so similar, that 
comparisons may be fairly made. It is true that, as hereafter 
to be shown, the differences of expenditure which differences 
of bulk entail, have doubtless much to do with the differences 
of fertility. But we may set down under the present head 
some of those cases in which the activity, being relatively 
slight, does not greatly interfere with the relation we are 
considering; and may note that among such birds having 
similarly slight activities, the small produce more eggs than 
the large, and eggs that bear in their total mass a greater 
ratio to the mass of the parent. Consider, for example, the 
gallinaceous birds; which are like one another and unlike 
birds of most other groups in flying comparatively little. 
Taking first the wild members of this order, which rarely breed 
more than once in a season, we find that the Pheasant has 
from 6 to 10 eggs, the Black-cock from 5 to 10, the Grouse 
8 to 12, the Partridge 10 to 15, the Quail still more, some- 
times reaching 20. Here the only exception to the relation 
between decreasing bulk and increasing number of eggs, 
occurs in the cases of the Pheasant and the Black-cock ; and 
it is to be remembered, in explanation, that the Pheasant 
inhabits a warmer region and is better fed—often artificially. 
If we pass to domesticated genera of the same order, we 
meet with parallel differences. From the numbers of eggs 
laid, little can be inferred; for under the favourable con- 
ditions artificially maintained, the laying is carried on inde- 
finitely. But though in the sizes of their broods the Turkey 
and the Fowl do not greatly differ, the Fowl begins breeding 
at a much earlier age than the Turkey, and produces 
broods more frequently: a considerably higher rate of 
multiplication being the result. Now these contrasts 


GROWTH AND SEXUAL GENESIS. 435 


among domestic creatures that are similarly conditioned, 
and closely-allied by constitution, may be held to’ show, 
more clearly than most other contrasts, the inverse varia- 
tion between bulk and sexual genesis; since here the 
cost of activity is diminished to a comparatively small 
amount. ‘There is little expenditure in flight—sometimes 
almost none; and the expenditure in walking about is 
not great: there is more of standing than of actual 
movement. It is true that young Turkeys commence 
their existences as larger masses than chickens; but it is 
tolerably manifest that the total weight of the eggs produced 
by a Turkey during each season, bears a less ratio to the 
Turkey’s weight, than the total weight of the eggs which a 
Hen produces during each season, bears to the Hen’s weight ; 
and this is the fairest way of making the comparison. The 
comparison so made shows a greater difference than appears 
likely to be due to the different costs of locomotion; con- 
sidering the inertness of the creatures. Remembering that 
the assimilating surface increases only as the squares of the 
dimensions, while the mass of the fabric to be built up by the 
absorbed nutriment increases as the cubes of the dimensions, 
it will be seen that the expense of growth becomes relatively 
greater with each increment of size; and that hence, of two 
similar creatures commencing life with different sizes, the 
larger one in reaching its superior adult bulk, will do this at 
a more than proportionate expense; and so will either be 
delayed in commencing its reproduction, or will have a 
diminished reserve for reproduction, or both. Other orders 
of Birds, active in their habits, show more markedly the con- 
nexion between augmenting mass and declining fertility. 
But in them the increasing cost of locomotion becomes an 
important, and probably the most important, factor. The 
evidence they furnish will therefore come better under 
another head. Contrasts among Mammals, like 
those which Birds present, have their meanings obscured by 


inequalities of the expenditure for motion. The smaller 
28 * 


436 LAWS OF MULTIPLICATION. 


fertility which habitually accompanies greater bulk, must 
in all cases be partly ascribed to this. Still, it may be 
well if we briefly note, for as much as they are worth, 
the broader contrasts. While a large Mammal bears but 
a single young one at a time, is several years before it 
commences doing this, and then repeats the reproduction at 
long intervals; we find, as we descend to the smaller mem- 
bers of the class, a very early commencement of breeding, an 
increasing number at a birth, reaching in small Rodents to 
10 or even more, and a much more frequent recurrence of 
broods: the combined result being a relatively prodigious 
fertility. If a specific comparison be desired between 
Mammals that are similar in constitution, in food, in con- 
ditions of life, and all other things but size, the Deer-tribe 
supplies it. While the large Red-deer has but one at a 
birth, the small Roe-deer has two at a birth. 


§ 341. The antagonism between growth and sexual genesis, 
visible in these general contrasts, may also be traced in the 
history of each plant and animal. So familiar is the fact 
that sexual genesis does not occur early in life, and in all 
organisms which expend much begins only when the limit ot 
size is nearly reached, that we do not sufficiently note its 
significance. It is a general physiological truth, however, 
that while the building-up of the individual is going on 
rapidly, the reproductive organs remain imperfectly developed 
and inactive; and that the commencement of reproduction 
at once indicates a declining rate of growth, and becomes a 
cause of arresting growth. As was shown in § 78, the ex- 
ceptions to this rule are found where the limit of growth is 
indefinite ; either because the organism expends little or 
nothing in action, or expends in action so moderate an 
amount that the supply of nutriment is never equilibrated 
by its expenditure. 

We will pass over the inferior plants, and limiting our- 
selves to Pheenogams, will not dwell on the less conspicu- 


GROWTH AND SEXUAL GENESIS. 437 


ous evidence which the smaller types present. A few cases 
suchas gardens supply will serve. All know that a Pear- 
tree continues to increase in size for years before it begins to 
bear ; and that, producing but few pears at first, it is long 
before it fruits abundantly. A young Mulberry, branch- 
ing out luxuriantly season after season, but covered 
with nothing but leaves, at length blossoms sparingly, and 
sets some small and imperfect. berries, which it drops while 
they are green; and it makes these futile attempts time after 
time before it succeeds in ripening any seeds. But these 
multi-axial plants, or aggregates of individuals some of 
which continue to grow while others become arrested and 
transformed into seed-bearers, show us the relation less de- 
finitely than certain plants that are substantially, if not 
literally, uni-axial. Of these the Cocoa-nut may be in- 
stanced. For some years it goes on shooting up without 
making any sign of becoming fertile. About the sixth year 
it flowers; but the flowers wither without result. In the 
seventh year it flowers and produces a few nuts; but these 
prove abortive and drop. In the eighth year it ripens a 
moderate number of nuts; and afterwards increases the 
number until, in the tenth year, it comes into full bearing. 
Meanwhile, from the time of its first flowering its growth 
begins to diminish, and goes on diminishing till the tenth 
year, when it ceases. Here we see the antagonism between 
growth and sexual genesis under both its aspects—see a 
struggle between self-evolution and race-evolution, in which 
the first for a time overcomes the last, and the last ultimately 
overcomes the first. The continued aggrandisement of the 
parent-individual makes abortive for two seasons the tendency 
to produce new individuals; and the tendency to produce 
new individuals, becoming more decided, stops any further 
agerandisement of the parent-individual. 

Parallel illustrations occur in the animal kingdom. The 
eggs laid by a pullet are relatively small and few. Similarly, 
it is alleged that, as a general rule, “a bitch has fewer 


438 LAWS OF MULTIPLICATION. 


puppies at first, than afterwards.” According to Burdach, 
as quoted by Dr. Duncan, “the elk, the bear, &c., have at 
first only a single young one, then they come to haye most 
frequently two, and at last again only one. The young 
hamster produces only from three to six young ones, whilst 
that of a more advanced age produces from eight to sixteen. 
The same is true of the pig.” It is remarked by Buffon that 
when a sow of less than a year old has young, the number of 
the litter is small, and its members are feeble and even im- 
perfect. Here we have evidence that in animals growth 
checks sexual genesis. And then, conversely, we have 
evidence that sexual genesis checks growth. It is well 
known to breeders that if a filly is allowed to bear a foal, 
she is thereby prevented from reaching her proper size. And 
a like loss of perfection as an individual, is suffered by a 
cow that breeds too early. 


§ 342. Notwithstanding: the way in which the inverse 
variation of growth and sexual genesis is complicated with 
other relations, its existence is thus, I think, sufficiently mani- 
fest. Individually, many of the foregoing instances are open 
to criticism, and have to be taken with qualifications; but 
when looked at in the mass, their meaning is beyond doubt. 
Comparisons between the largest with the smallest types, 
whether vegetal or animal, yield results that are unmis- 
takeable. On the one hand, remembering the fact that 
during its centuries of life an Oak does not produce as many 
acorns as a F'ungus does spores in a single night, we see that 
the Fungus has a fertility exceeding that of the Oak in a de- 
gree literally beyond our powers of calculation or imagina- 
tion. When, on the other hand, taking a microscopic 
protophyte which has millions of descendants in a few days, 
we ask how many such would be required to build up the 
forest tree that is years before it drops a seed, we are met 
by a parallel difficulty in conceiving the number, if not in 
setting it down. Similarly, if we turn from the minute and 


GROWTH AND SEXUAL GENESIS. 439 


prodigiously-fertile Rotifer, to the Elephant, which approaches 
thirty years before it bears a solitary young one, we find the 
connexions between small size and great fertility and between 
great size and small fertility, too intensely marked to be 
much disguised by the perturbing relations that have been 
indicated. Finally, as this induction, reached by a survey of 
organisms in general, is verified by observations on the rela- 
tion between decreasing growth and commencing reproduc- 
tion in individual organisms, we may, I think, consider the 
alleged antagonism as proved.* 


* When, after having held for some years the general doctrine elaborated in 
these chapters, I agreed, early in 1852, to prepare an outline of it for the West- 
minster Review, I consulted, among other works, the just-issued third edition 
of Dr. Carpenter’s Principles of Physiology, General and Comparative—secking 
in it for facts illustrating the different degrees of fertility of different organisms. 
I met with a passage, quoted above in §339, which seemed tacitly to assert 
that individual aggrandizement is at variance with the propagation of the race; 
but nowhere found a distinct enunciation of this truth. I did not then read 
the Chapter entitled ‘‘General View of the Functions,” which held out 
no promise of such evidence as I was looking for. But on since referring to 
this chapter, I discovered in it the definite statement that—“‘ there is a certain 
degree of antagonism between the Nutritive and Reproductive functions, the 
one being executed at tne expense of the other. The reproductive apparatus 
derives the materials of its operations through the nutritive system, and 
is entirely dependent upon it for the continuance of its function. If, there- 
fore, it be in a state of excessive activity, it will necessarily draw off from the 
individual fabric some portion of the aliment destined for its maintenance. 
It may be universally observed that, when the nutritive functions are 
particularly active in supporting the individual, the reproductive system is in 
a corresponding degree undeveloped,—and vice versd.” —Principles of Phy- 
siology, General and Comparative, Third Edition, 1851, p. 592, 


CHAPTER VIl. 


THE ANTAGONISM BETWEEN DEVELOPMENT AND GENESIS, 
ASEXUAL AND SEXUAL. 


§ 343. By Development, as here to be dealt with apart 
from Growth, is meant increase of structure as distinguished 
from increase of mass. As was pointed out in § 50, this is 
the biological definition of the word. In the following 
sections, then, we have to note how complexity of organiza- 
tion is hindered by reproductive activity, and conversely. 

This relation partially coincides with that which we have 
just contemplated; for, as was shown in § 44, degree of 
growth is to a considerable extent dependent on degree of 
organization. But while the antagonism to be illustrated in 
this chapter, is much entangled with that illustrated in the 
last chapter, it may be so far separated as to be identified as 
an additional antagonism. 

Besides the direct opposition between that continual dis- 
integration which rapid genesis implies, and the fulfilment of 
that pre-requisite to extensive organization—the formation of 
an extensive aggregate, there is an indirect opposition which 
we may recognize under several aspects. The change 
from homogeneity to heterogeneity takes time; and time taken 
in transforming a relatively-structureless mass into a de- 
veloped individual, delays the period of reproduction. Usually 
this time is merged in that taken for growth; but certain 
cases of metamorphosis show us the one separate from the 


DEVELOPMENT AND GENESIS. 44] 


other. An insect, passing from its lowly-organized cater- 
pillar-stage into that of chrysalis, is afterwards a week, a fort- 
night, or a longer period in completing its structure: the re- 
commencement of genesis being by so much postponed, and 
the rate of multiplication therefore diminished. Further, that 
re-arrangement of substance which development implies, en- 
tails expenditure. The chrysalis loses weight in the course 
of its transformation; and that its loss is not loss of water 
only, may be inferred from the fact that it respires, and that 
respiration indicates consumption. Clearly the matter con- 
sumed, is, other things equal, a deduction from the surplus 
that may go to reproduction. Yet again, the 
more widely and completely an organic mass becomes diffe- 
rentiated, the smaller the portion of it which retains the re- 
latively-undifferentiated state that admits of being moulded 
into new individuals, or the germs of them. Protoplasm 
which has become specialized tissue, cannot be again 
generalized, and afterwards transformed into something else ; 
and hence the progress of structure in an organism, by 
diminishing the unstructured part, diminishes the amount 
available for making offspring. 

It is true that higher structure, like greater growth, may 
insure to a species advantages that eventually further its mul- 
tiplication—may give it access to larger supplies of food, or 
enable it to obtain food more economically; and we shall 
hereafter see how the inverse variation we are considering is 
thus qualified. But here we are concerned only with the 
necessary and direct effects; not with those that are con- 
tingent and remote. These necessary and direct effects we 
will now look at as exemplified. 


§ 344. Speaking generally, the simpler plants propagate 
both sexually and asexually; and, speaking comparatively, 
the complex plants propagate only sexually: their asexual 
propagation is usually incomplete—produces a united aggre- 
gate of individuals instead of numerous distinct individuals 


442 LAWS OF MULTIPLICATION. 


The Protophytes that perpetually subdivide, the merely- 
cellular Alge that shed their tetraspores, the Acrogens that 
spontaneously separate their fronds and drop their gemma, 
show us an extra mode of multiplication which, among flower- 
ing plants, is exceptional. This extra mode of multiplication 
among these simpler-plants, is made easy by their low de- 
velopment. Tetraspores arise only where the frond consists 
of untransformed cells; gemme bud out and drop off only 
where the tissue is comparatively homogeneous. 

Should it be said that this is but another aspect of the 
antagonism already set forth, since these undeveloped forms 
are also the smaller forms; the reply is that though in part 
true, this is not wholly true. Various marine Alge which 
propagate asexually, are larger than some Pheenogams which 
do not thus propagate. The objection that difference of 
medium vitiates this comparison, is met by the fact that it is 
the same among land-plants themselves. Sundry of the 
lowly-organized Liverworts that are habitually gemmiparous, 
exceed in size many flowering plants. And the Ferns show 
us agamic multiplication occurring in plants which, while 
they are inferior in complexity of structure, are superior in 
bulk to a great proportion of annual Endogens and Exogens. 


§ 345. In the ability of the lowly-organized, or almost 
unorganized, sarcode of a Sponge, to transform itself into 
multitudes of gemmules, we have an instance of this same 
direct relation in the animal kingdom. Moreover, the 
instance yields very distinct proof of an antagonism between 
development and genesis, independent of the antagonism 
between growth and genesis; for the Sponge which thus 
multiplies itself asexually, as well as sexually, is far larger 
than hosts of more complex animals which do not multiply 
asexually. 

Once again may be cited the creature so often brought in 
evidence, the Hydra, as showing us how rapidity of agamic 
propagation is associated with inferiority of structure. Its 


DEVELOPMENT AND GENESIS. 443 


power to produce young ones from nearly all parts of its 
body, is due to the comparative homogeneity of its body. In 
kindred but more-organized types, the gemmiparity is 
greatly restricted, or disappears. Among the free-swimming 
Hydrazoa, multiplication by budding, when it occurs at all, 
occurs only at special places. ‘That increase of structure 
apart from increase of size, is here a cause of declining agamo- 
genesis, we may see in the contrast between the simple and 
the compound Hydroida; which last, along with more- 
differentiated tissues, show us a gemmation which does not 
go on all over the body of each polype, and much of which 
does not end in separation. 

It is, however, among the Annulosa that progressing 
organization is most conspicuously operative in diminishing 
agamogenesis. The segments or “somites” that compose an 
animal belonging to this class, are primordially alike; and, 
as before argued (§§ 205-7), are probably the homologues of 
what were originally independent individuals. The progress 
from the lower to the higher types of the class, is at once a 
progress towards types in which the strings of segments cease 
to undergo subdivision, and towards types in which the seg- 
ments, no longer alike in their structures and functions, have 
become physiologically integrated or mutually dependent. 
Already this group of cases has been named as illustrating 
the antagonism between growth and asexual genesis; but it 
is proper also to name it here; since, on the one hand, the 
greater size due to the ceasing of fission, is made possible only 
by the specialization of parts and the development of a co- 
ordinating apparatus to combine their actions, and since, on 
the other hand, specialization and co-ordination can advance 
only in proportion as fission ceases. 


§ 346. The inverse variation of development and sexual 
genesis is by no means easy to follow. One or two facts indi 
cative of it may, however, be named. 

Phenogams that have but little supporting tissue may 


444 LAWS OF MULTIPLICATION. 


fairly be classed as structurally inferior to those provided with 
stems formed of woody fibres; for these imply additional dif- 
ferentiations, and constitute wider departures from the primi- 
tive type of vegetal tissue. That the concomitant of this 
higher organization is a slower gamogenesis, scarcely needs 
pointing out. While the herbaceous annual is blossoming 
and ripening seed, the young tree is transforming its ori- 
ginally-succulent axis into dense fibrous substance ; and year 
by year the young tree expends in doing the like, nutriment 
which successive generations of the annual expend in fruit. 
Here the inverse relation is between sexual reproduction and 
complexity, and not between sexual reproduction and bulk 
seeing that besides seeding, the annual often grows to a size 
greater than that reached by the young infertile tree in 
several years. 

Proof of the antagonism between complexity and gamo- 
genesis in animals, is still more difficult to disentangle. Per- 
haps the evidence most to the point is furnished by the contrast 
between Man and certain other Mammals approaching to him | 
in mass. To compare him with the domestic Sheep, which, 
though not very unlike in size, is relatively prolific, is objec- 
jectionable because of the relative inactivity of Sheep; and 
this, too, may be alleged as a reason why the Ox, though far 
more bulky, is also far more fertile, than Man. Further, 
against a comparison with the Horse, which, while both larger 
and more prolific, is tolerably active, it may be urged that, in 
his case, and the cases of herbivorous creatures generally, the 
small exertion required to procure food, joined with the great 
ratio borne by the assimilative organs to the organs they have 
to build up and repair, vitiates the result. We may, however, 
fairly draw a parallel between Man anda large carnivore. The 
Lion, superior in size, and perhaps equal in activity, has a 
digestive system not proportionately greater; and yet has a 
higher rate of multiplication than Man. Here the only de- 
cided want of parity, besides that:of organization, is that of 
food. Possibly a carnivore gains an advantage in having a 


DEVELOPMENT AND GENESIS. 445 


surplus nutriment consisting almost wholly of those nitro- 
genous materials from which the bodies of young ones are 
mainly formed. But, allowing for all other differences, it 
appears not improbable that the smallness of human fertility 
compared with the fertility of large feline animals, is due to 
the greater complexity of the human organization —more 
especially the organization of the nervous system Taking 
degree of nervous organization as the chief correlative of 
mental capacity ; and remembering the physiological cost of 
that discipline whereby high mental capacity is reached; we 
may suspect that nervous organization is very expensive : the 
inference being that bringing it up to the level it reaches in 
Man, whose digestive system, by no means large, has at the 
same time to supply materials for general growth and daily 
waste, involves a’ great retardation of maturity and sexual 
genesis. 


CHAPTER VIII. 
ANTAGONISM BETWEEN EXPENDITURE AND GENESIS. 


§ 347. Under this head we have to set down no evidence 
derived from the vegetal kingdom. Plants are not expenders 
of force in such degrees as to affect the general relations with 
which we are dealing. Thev have not to maintain a heat 
above that of their environment; nor have they to generate 
motion; and hence consumption for these two purposes. does 
not diminish the stock of material that serves on the one 
hand for growth and on the other hand for propagation. 

It will be well, too, if we pass over the lower animals: 
especially those aquatic ones which, being nearly of the 
same temperature as the water, and nearly of the same 
specific gravity, lose but little in evolving motion, sensible 
and insensible. A further reason for excluding from con- 
sideration these inferior types, is, that we do not know enough 
of their rates of genesis to permit of our making, with any 
satisfaction, those involved comparisons here to be entered 
upon. 

The facts on which we must mainly depend are those to be 
gathered from terrestrial animals; and chiefly from those 
higher classes of them which are at the same time great 
expenders and have rates of multiplication about which our 
knowledge is tolerably definite. We will restrict ourselves, 
then, to the evidence which Birds and Mammals supply. 


§ 348. Satisfactory proof that loss of substance in the 


EXPENDITURE AND GENKESIS. 447 


maintenance of heat diminishes the rapidity of propagation, 
is difficult to obtain. It is, indeed, obvious that the warm- 
blooded Vertebrata are less prolific than the cold-blooded ; 
but then they are at the same time more vivacious. Similarly, 
between Mammala and Birds (which are the warmer-blooded 
of the two) there is, other things equal, a parallel, though 
much smaller, difference; but here, too, the unlikenesses of 
muscular action complicate the evidence. Again, the annual 
return of generative activity has an average correspondence 
with the annual return of a warmer season, which, did it 
stand alone, might be taken as evidence that a diminished 
cost of heat-maintenance leads to such a surplus as makes 
reproduction possible. But then, this periodic rise of tem- 
perature is habitually accompanied by an increase in the 
quantity of food—a factor of equal or greater importance. 
We must be content, therefore, with such few special facts 
as admit of being disentangled. 

Certain of these we are introduced to by the general rela- 
tion last named—the habitual recurrence of genesis with the 
recurrence of spring. Jor in some cases a domesticated crea- 
ture has its supplies of food almost equalized; and hence the 
effect of varying nutrition may bein great part eliminated 
from the comparison. The common Fowl yields an illustra- 
tion. It is fed through the cold months, but nevertheless, in 
mid-winter, it either wholly leaves off laying or lays very 
sparingly. And then we have the further evidence that if it 
lays sparingly, it does so only on condition that the heat, as 
well as the food, is artificially maintained. Hens lay in cold 
weather only when they are kept warm. To which fact may 
be added the kindred one that ‘“ when pigeons receive arti- 
ficial heat, they not only continue to hatch longer in autumn, 
but will recommence in spring sooner than they would other- 
wise do.” An analogous piece of evidence is that, in 
winter, inadequately sheltered Cows either cease to give milk 
or give it in diminished quantity. For though giving milk 
is not the same thing as bearing a young one, yet, as milk 


448 LAWS OF MULTIPLICATION. 


is part of the material from which a young one is built up, 
it is part of the outlay for reproductive purposes, and diminu- 
tion of it is a loss of reproductive power. Indeed the case 
aptly illustrates, under another aspect, the struggle between 
self-preservation and race-preservation. Maintenance of the 
cow’s life depends on maintenance of its heat; and main- 
tenance of its heat may entail such reduction in the supply 
of milk as to cause the death of the calf. 

Evidence derived from the habits of the same or allied 
genera in different climates, may naturally be looked for; but 
it is difficult to get, and it can scarcely be expected that the 
remaining conditions of existence will be so far similar as to 
allow of a fair comparison being made. The only illustrative 
facts I have met with which seem noteworthy, are some named 
by Mr. Gould in his work on The Birds of Australia. He 
says :—‘‘ I must not omit to mention, too, the extraordinary 
fecundity which prevails in Australia, many of its smaller 
birds breeding three or four times in a season; but laying 
fewer eggs in the early spring when insect life is less 
developed, and a greater number later in the season, when 
the supply of insect food has become more abundant. I have 
also some reason to believe that the young of many species 
breed during the first season, for among others, I frequently 
found one section of the Honey-eaters (the MMelithrepti) 
sitting upon eggs while still clothed in the brown dress of 
immaturity ; and we know that such is the case with the 
introduced G'adlinacee (or poultry) three or four generations 
of which have been often produced in the course of a year.” 
Though here Mr. Gould refers only to variation in the 
quantity of food as a cause of variation in the rate of 
multiplication, may we not suspect that the warmth is a 
part-cause of the high rate which he describes as general P 


§ 349. Of the inverse variation between activity and 
genesis, we get clear proof. Let us begin with that which 
Birds furnish. 


EXPENDITURE AND GENESIS. 449 


First we have the average contrast, already hinted, between 
the fertility of Birds and the fertility of Mammals. Compar- 
ing the large with the large and the small with the small, we 
see that creatures which continually go through the muscular 
exertion of sustaining themselves in the air and propelling 
themselves rapidly through it, are less prolific than creatures 
of equal weights which go though the smaller exertion of 
moving about over solid surfaces. Predatory Birds have 
fewer young ones than predatory Mammals of approximately 
the same sizes. If we compare Rooks with Rats, or Finches 
with Mice, we find like differences. And these differences are 
greater than at first appears. For whereas among Mammals 
a mother is able, unaided, to bear and suckle and rear half- 
way to maturity, a brood that probably weighs more in pro- 
portion than does the brood of a Bird; a Bird, or at least a 
Bird that flies much, is unable to do this. Both parents have 
to help; and this indicates that the margin for reproduction 
in each adult individual is smaller. 

Among Birds themselves occur contrasts which may be 
next considered. In the Raptorial class, various species of 
which, differing in their sizes, are similarly active in their 
habits, we see that the small are more prolific than the large. 
The Golden Eagle has usually 2 eggs: sometimes only 1. 
As we descend to the Kites and Falcons, the number is 2 or 
or 3, and 3 or 4. And when we come to the Sparrow-Hawk, 
3 to 5 is the specified number. Similarly among the Owls: 
while the Great Hagle-Owl has 2 or 3 eggs, the comparatively 
small Common Owl has 4 or 5. As before hinted, it is im- 
possible to say what proportions of these differences are due 
to unlikenesses of bulk merely, and what proportions are due 
to unlikenesses in the costs of locomotion. But we may fairly 
assume that the unlikenesses in the costs of locomotion are 
here the more important factors. Weights varying as the 
cubes of the dimensions, while muscular powers vary as the 
squares, the expense of flight increases more rapidly than the 


size increases; and as motion through the air requires more 
VOL. II. 29° | 


450 -LAWS OF MULTIPLICATION. 


effort than motion on the ground, this geometrical progression 
tells more rapidly on Birds than on Mammals. Be this as it 
may, however, these contrasts support the argument; as do 
various others that may be setdown. The Finch family, for 
example, have broods averaging about 5 in number, and have 
commonly 2 broods in the season; while in the Crow family 
the number of the brood is on the average less, and there is 
but one brood ina season. And then on descending to such 
small birds as the Wrens and the Tits, we have 8, 10, 12 to 
15 eggs, and often two broods in the year. One of the best 
illustrations is furnished by the Swallow-tribe, throughout 
which there is little or no difference in mode of life or in food. 
The Sand-Martin, much the least of them, has usually 6 eggs; 
the Swallow, somewhat larger, has 4 or 5; and the Swift, 
larger still, has but 2. Here we see a lower fertility associated 
in part with greater size, but associated still more con- 
spicuously with greater expenditure. or the difference of 
fertility is more than proportionate to the difference of bulk, 
as shown in other cases; and for this greater difference there 
is the reason, that the Swift has to support not only the cost 
of propelling its larger mass through the air, but also the cost 
of propelling it at a higher velocity. 

Omitting much evidence of like nature, let us note that 
disclosed by comparisons of certain groups of birds with other 
groups. ‘‘Skulkers” is the descriptive title applied to the 
Water-Rail, the Corn-Crake, and their allies, which evade 
enemies by concealment—consequently expending but little 
in locomotion. These birds have relatively large broods—6 
to 11, 8 to 12, &c. Not less instructive are the contrasts be- 
tween the Gallinaceous Birds and other Birds of like sizes but 
more active habits. The Partridge and the Wood-Pigeon are 
about equal in bulk, and have much the same food. Yet while 
the one has from 10 to 15 young ones, the other has but 2 
young ones twice a-year: its annual reproduction is but 
one-third. It may be said that the ability of the Partridge 
to bring up so large a brood, is due to that habit of its tribe 


EXPENDITURE AND GENESIS. 451 


which one of its names, ‘“ Scrapers,” describes; and to the 
accompanying habit of the young, which begin to get their 
own living as soon as they are hatched: so saving the parents’ 
labour. Conversely, it may be said that the inability of the 
Pigeon to rear more than 2 at a time, is caused by the necessity 
of fetching everything they eat. But the alleged relation 
holds nevertheless. On the one hand, a great part of the food 
which the Partridge chicks pick up, is food which, in their 
absence, the mother would have picked up: though each chick 
costs her far less than a young Pigeon costs its parents, yet 
the whole of her chicks cost her a great deal in the shape of 
abstinence—an abstinence she can bear because she has to fly 
but little. On the other hand, the Pigeon’s habit of laying 
and hatching but two eggs, must not be referred to any fore- 
seen necessity of going through so much labour in supporting 
the young, but to a constitutional tendency established by such 
labour. This is proved by the curious fact that when do- 
mesticated, and saved from such labour by artificial feeding, 
Pigeons, says Macgillivray, ‘‘are frequently seen sitting on 
egos long before the former brood is able to leave the nest, so 
that the parent bird has at the same time young birds and 
egos to take care of.” 


§ 350. Made to illustrate the effect of activity on fertility, 
most comparisons among Mammals are objectionable: other cir- 
cumstances are not equal. A few, however, escape this criticism. 

One is that between the Hare and the Rabbit. These are 
closely-allied species of the same genus, similar in their diet 
but unlike in their expenditures for locomotion. The rela- 
tively-inert Rabbit has 5 to 8 young ones in a litter, and 
several litters a-year; while the relatively-active Hare has 
but 2 to 5 in a litter. This is not all. The Rabbit begins 
to breed at six months old; but a year elapses before the 
Hare begins to breed. These two factors compounded, result 
in a difference of fertility far greater than can be ascribed to 


unlikeness of the two creatures in size. 
av Psd 


452 _ LAWS OF MULTIPLICATION. 


Perhaps the most striking piece of evidence which Mam- 
mals furnish, is the extreme infertility of our common Bat. 
The Cheiroptera and the Rodentia are very similar in their 
internal structures. Diversity of constitution, therefore, 
cannot vitiate the comparison between Bats and Mice, which 
are about the same in size. Though their diets differ, the 
difference is in favour of the Bat: its food being exclusively 
animal while that of the Mouse is mainly vegetal. What 
now are their respective rates of genesis? The Mouse pro- 
duces many young at a time, reaching even 10 or 12; while 
the Bat produces only one at a time. Whether the Bat 
repeats its one more frequently than the Mouse repeats its 
ten is not stated; but it is quite certain that even if it does 
so, the more frequent repetition cannot be such as to raise its 
fertility to anything like that of the Mouse. And this 
relatively-low rate of multiplication we may fairly ascribe to 
its relatively-hizh rate of expenditure. 

Here let us note, in passing, an interesting example of the 
way in which a species that has no specially-great power of 
self-preservation, while its power of multiplication is extremely 
small, nevertheless avoids extinction because it has to meet 
an unusually-small total of race-destroying forces. Leaving 
out parasites, the only enemy of the Bat is the Owl; and the 
Owl is sparingly distributed. 


§ 3851. These general evidences may be enforced by some 
special evidences. We have few opportunities of observing 
how, within the same species, variations of expenditure are 
related to variations of fertility. But a fact or two showing 
the connexion may be named. 

Doctor Duncan quotes a statement to the point respecting 
the breeding of dogs. Already in § 341 I have extracted a part 
of this statement, to the effect that before her growth is com- 
plete, a bitch bears at a birth fewer puppies than when she 
becomes full-grown. An accompanying allegation is, that 
her declining vigour is shown by a decrease in the number of 


EXPENDITURE AND GENESIS. 453 


puppies contained in a litter, ‘‘ ending in one or two.” And 
then it is further alleged that, “as regards the amount of 
work a dog has to perform, so will the decline be rapid or 
gradual ; and hence, if a bitch is worked hard year after year, 
she will fail rapidly, and the diminution of her puppies will 
be accordingly ; but if worked moderately and well kept, she 
will fail gradually, and the diminution will be less rapid.” 

In this place, more fitly than elsewhere, may be added a 
fact of like implication, though of a different order. Of course 
whether excessive expenditure be in the continual repairs of 
nervo-muscular tissues or in replacing other tissues, the re- 
active effects, if not quite the same, will be similar—there 
will be a decrease of the surplus available for genesis. If, 
then, in any animals there from time to time occur unusual 
outlays for self-maintenance, we may expect the periods of 
such outlays to be periods of diminished or arrested repro- 
duction. That they are so the moulting of birds shows us. 
When hens begin to moult they cease to lay. While they 
are expending so much in producing new clothing, they have 
nothing to expend for producing eggs. 


CHAPTER IX. 
COINCIDENCE BETWEEN HIGH NUTRITION AND GENESIS. 


§ 852. Under this head may be grouped various facts 
which, in another way, tell the same tale as those contained 
in the last chapter. The evidence there put together went to 
show that increased cost of self-maintenance entailed de- 
creased power of propagation. The evidence to be set down 
here, will go to show that power of propagation is augmented 
by making self-maintenance unusually easy. For into this 
may be translated the effect of abundant food. 

To put the proposition more specifically—we have seen 
that after individual growth, development, and daily con- 
sumption have been provided for, the surplus nutriment 
measures the rate of multiplication. This surplus may be 
raised in amount by such changes in the environment as 
bring a larger supply of the materials or forces on which 
both parental life and the lives of offspring depend. Be 
there, or be there not, any expenditure, a higher nutrition 
will make possible a greater propagation. We may expect 
this to hold both of agamogenesis and of gamogenesis; and 
we shall] find that it does so. 


§ 3538. On multi-axial plants, the primary effect of surplus 
nutriment is a production of large and numerous leaf-shoots. 
How this asexual multiplication results from excessive nutri- 
tion, is well shown when the leading axis, ora chief branch, is 
broken off towards its extremity. The axillary buds below 


NUTRITION AND GENESIS. 455. 


the breakage quickly swell and burst into lateral shoots, 
which often put forth secondary shoots: two generations of 
agamic individuals arise where there probably would have 
been none but for the local abundance of sap, no longer 
drawn off. In like manner the abnormal agamogenesis which 
we have in proliferous flowers, is habitually accompanied by 
a general luxuriance, implying an unusual plethora. 

No less conclusive is the evidence furnished by agamo- 
genesis in animals. Sir John Dalyell, speaking of Hydra 
tuba, whose peculiar metagenesis he was the first to point out, 
says—“ It is singular how much propagation is promoted by 
abundant sustenance.” This Polype goes on budding-out young 
polypes from its sides, with a rapidity proportionate to the 
supply of materials. So, too, is 1 with the agamic 
reproduction of the Aphis. As cited by Professor Huxley, 
Kyber “ states that he raised viviparous broods of both this 
species ( Aphis Dianthi) and A. Rose for four consecutive 
years, without any intervention of males or oviparous females, 
and that the energy of the power of agamic reproduction was 
at the end of that period undiminished. The rapidity of the 
agamic prolification throughout the whole period was directly 
proporticnal to the amount of warmth and food supplied.” 

In these cases the relation is not appreciably complicated by 
expenditure. The parent having reached its limit of growth, 
the absorbed food goes to asexual multiplication: scarcely 
any being deducted for the maintenance of parental life. 


§ 354. The sexual multiplication of organisms under 
changed conditions, undergoes variations conforming to a 
parallel law. Cultivated plants and domesticated animals 
yield us proof of this. | 

Facts showing that in cultivated plants, sexual genesis in- 
creases with nutrition, are obscured by facts showing that a 
less rapid asexual genesis, and an incipient sexual genesis, ac- 
company the fall from a high to a moderate nutrition. The 
confounding of these two relations has led to mistaken infer- 


456 LAWS OF MULTIPLICATION 


ences. When treating of Genesis inductively, we reached the 
generalization that ‘‘ the products of a fertilized germ go 
on accumulating by simple growth, so long as the forces 
whence growth results are greatly in excess of the antagonist 
forces; but that when diminution of the one set of forces, or 
increase of the other, causes a considerable decline in this ex- 
cess, and an approach towards equilibrium, fertilized germs 
are again produced.” (§ 78.) It was pointed out that this 
holds of organisms which multiply by heterogenesis, as 
well as those which multiply by homogenesis. And plants 
were referred to as illustrating, both generally and locally, 
the decline of agamic multiplication and commencement of 
gamic multiplication, along with a lessening rate of nutrition. 
Now the many cases that are given of fruitfulness caused in 
trees by depletion, are really cases of this change from 
agamogenesis to gamogenesis; and simply go to prove that 
what would naturally arise when decreased peripheral growth 
had followed increased size, may be brought about artificially 
by diminishing the supply of materials for growth. Cramp-_ 
ing its roots in a pot, or cutting them, or ringing its branches, 
will make a tree bear very early: bringing about a pre- 
mature establishment of that relative innutrition which 
would have spontaneously arisen in course of time. Such 
facts by no means show that in plants, sexual genesis in- 
creases as nutrition diminishes. When it has once set in, 
sexual genesis is scanty or imperfect unless nutrition is good. 
Though the starved plant may blossom, yet many of its 
blossoms will fail; and such seeds as it produces will be ill- 
furnished with those enveloping structures and that store of 
albumen, &c., needed to give good chances of successful germi. 
nation—the number of surviving offspring will be diminished. 
Were it otherwise, the manuring of fields that are to bear 
seed-crops, would be not simply useless but injurious. Were 
it otherwise, dunging the roots of a fruit-tree would in all 
cases be impolitic; instead of being impolitic only where the 
growth of sexless axes is still luxuriant. Were it otherwise, 


NUTRITION AND GENESIS 457 


a tree which has borne a heavy crop, should, by the con- 
sequent depletion, be led to bear a still heavier crop next’ 
year; whereas it is apt to be wholly or partially barren next 
year—has to recover a state of tolerably-high nutrition: 
before its sexual genesis again becomes large. 

But the best illustrations are those yielded by animals, 
in which we have, besides an increased supply of nutriment, 
a diminished expenditure. ‘Two classes of comparisons, alike 
in their implications, may be made—comparisons between 
tame and wild animals of the same species or genus, and com- 
parisons between tame animals of the same species differently 
treated. 

To begin with Birds, let us first contrast the farm-yard 
Gallinacee with their kindred of the fields and woods. Not- 
withstanding their greater size, which, other things equal, 
should be accompanied by smaller fertility, the domesticated. 
kinds have more numerous offspring than the wild kinds. A 
Turkey has a dozen in a brood, while a Pheasant has from 6 
to 10. Twice or thrice in a season, a Hen rears as many 
chickens as a Partridge rears once in a season. Anserine birds 
show us parallel differences. The Tame Goose sits on 12 or 
more eggs, but the Wild Goose sits on 5, 6, or 7; and these 
are noted as considerably smaller. It is the same with Ducks: 
the domesticated variety lays and hatches twice as many eggs 
as the wild variety. And the like holds of Pigeons. After 
remarking of the Columba divia that “in spring when they 
have plenty of corn to pick from the newly-sown fields, they 
begin to get fat and pair; and again, in harvest, when the 
corn is cut down,” Macgillivray goes on to say, that ‘the 
same pair when tamed generally breed four times” in the 
year That between different poultry-yards, in- 
equalities of fertility are caused by inequalities in the supplies 
of food, is a familiar truth. High feeding shows its effects not 
only in the continuous laying, but also in the sizes of the 
eggs. Among directions given for obtaining eggs from 
pullets late in the year, it is especially insisted on that they 


458 LAWS OF MULTIPLICATION, 


shall havea generous diet. Respecting Pigeons Macgillivray 
writes :—‘‘that their breeding depends much on their 
having plenty of food to fatten them, seems, I think, 
evident from the circumstance that, when tamed, which 
they easily are, they are observed to breed in every month of 
the year. Ido not mean that the same pair will breed every 
month; but some in the flock, if well fed, will breed at any 
season.” There may be added a fact of like meaning 
which partially-domesticated birds yield. The Sparrow is one 
of the Finch tribe that has taken to the neighbourhood of 
houses; and by its boldness secures food not available to its 
congeners. The result is that it has several broods in a sea- 
son, while its field-haunting kindred have none of them more 
than two broods, and some have only one. 

Equally clear proof that abundant nutriment raises the rate 
of multiplication, occurs among Mammals. Compare the 
litters of the Dog with the litters of the Wolf and the Fox. 
Whereas those of the one range in number from 6 to 14, the 
others contain respectively 5 or 6 or occasionally 7, and 
4 or dor rarely 6. Again, the wild Cat has 4 or 5 kittens ; 
but the tame Cat has 5 or 6 kittens 2 or 3 times a-year. 
So, too, is it with the Weasel tribe. The Stoat has 5 
young ones once a-year. The Ferret has 2 litters yearly, 
each containing from 6 to 9; and this notwithstanding that 
it is the larger of the two. Perhaps the most striking 
contrast is that between the wild and tame varieties of 
the Pig. While the one produces, according to its age, from 
4 to 8 or 10 young ones, once a year, the other produces 
sometimes as many as 17 in a litter; or, in other cases, will 
bring up 6 litters of 10 each in two years—a rate of reproduc- 
tion that is unparalleled in animals of as large a size. 
And let us not omit to note that this excessive fertility 
occurs where there is the greatest inactivity—where there is 
plenty to eat and nothing to do. There is 
no less distinct evidence that among domesticated Mammals 
themselves, the well-fed individuals are more prolific than 


NUTRITION AND GENESIS. 459 


the ill-fed individuals. On the high and comparatively- 
infertile Cotswolds, it is unusual for Ewes to have twins; but 
they very commonly have twins in the adjacent rich valley of 
the Severn. Similarly, among the barren hills of the west of 
Scotland, two lambs will be borne by about one Ewe in twenty ; 
whereas in England, something like one Ewe in three will 
bear two lambs. Nay, in rich pastures, twins are more 
frequent than single births: and it occasionally happens 
that, after a genial autumn and consequent good grazing, a 
flock of Ewes will next spring yield double their number of 
Jambs—the triplets balancing the unipare. So direct 1s this 
relation, that I have heard a farmer assert his ability to fore- 
tell, from the high, medium, or low, condition of an Ewe in 
the autumn, whether she will next spring bear two, or one, 
or none. 


~§ 355. An objection must here be met. Many facts may 
be brought to prove that fatness is not accompanied by ferti- 
lity but by barrenness ; and the inference drawn is that high 
feeding is unfavourable to genesis. The premiss may be 
admitted while the conclusion is denied. 

There is a distinction between what may be called normal 
plethora, and an abnormal plethora, liable to be confounded 
with it. The one is a mark of constitutional wealth; but the 
other is a mark of constitutional poverty. Normal plethora 
is a superfluity of materials both for the building up of 
tissue and the evolution of force; and this is the plethora 
which we have found to be associated with unusual fecundity. 
Abnormal plethora, which, as truly alleged, is accompanied 
by infecundity, is a superfluity of force-evolving materials 
joined with either a positive or a relative deficiency of tissue- 
forming materials: the increased bulk indicating this state, 
being really the bulk of so much inert or dead matter. Note, 
first, a few of the facts which show us that obesity implies 
physiological impoverishment. 

Neither in brutes nor men does it ordinarily occur either 


460 LAWS OF MULTIPLICATION. 


in youth or in that early maturity during which the vigour 
is the greatest and the digestion the best: it does not 
habitually accompany the highest power of taking up nutri- 
tive materials. When fatness arises in the prime of life, 
whether from peculiarity of food or other circumstance, it is 
not the sign of an increased total vitality. On the contrary, if 
creat muscular action has to be gone through, the fat must 
be got rid of—either, as in a man, by training, or as in a 
horse that has grown bulky while out at grass, by putting 
him on such more nutritive diet as oats. The 
frequency of senile fatness, both in domesticated creatures 
and in ourselves, has a similar implication. Whether we 
consider the smaller ability of those who display it to with- 
stand large demands on their powers, or whether we consider 
the comparatively-inferior digestion common among them, 
we see that the increased size indicates, not an abundance of 
materials which the organism requires, but an abundance of 
materials which it does not require. Of like mean- 
ing is the fact that women who have had several children, 
and animals after they have gone on bearing young for some 
time, frequently become fat; and lose their fecundity as 
they do this. In such cases, the fatness is not to be taken as 
the cause of the infecundity; but the constitutional ex- 
haustion which the previous production of offspring has left, 
shows itself at once in the failing fecundity and the com- 
mencing fatness. There is yet another kind of evidence. 
Obesity not uncommonly sets in after the system has 
been subject to debilitating influences. Often a serious illness 
is followed by a corpulence to which there was previously no 
tendency. And the prolonged administration of mercury, con- 
stitutionally injurious as it 1s, sometimes produces a like effect. _ 

Closer inquiry verifies the conclusion to which these facts 
point. The microscope shows that along with the increase of 
bulk common in advanced life, there goes on what is called - 
“fatty degeneration :” oil-globules are deposited where there 
should be particles of flesh—or rather, we may say, the hydro- 


NUTRITION AND GENESIS. 461 


carbonaceous molecules locally produced by decomposition of 
the nitrogenous molecules, have not been replaced by other 
nitrogenous molecules, as they should have been. This fatty 
degeneration is, indeed, a kind of local death. For so regard- 
‘Ing it we have not simply the reason that an active substance 
has its place occupied by an inert substance; but we have 
the reason that the flesh of dead bodies, under certain 
conditions, is transformed into a fatty matter called adipocere. 

The infertility that accompanies fatness in domestic animals, 
has, however, other causes than that declining constitutional 
vigour which the fatness indicates. Being artificially fed, these 
animals cannot always obtain what their systems need. That 
which is given to them is often given expressly because of its 
fattening quality. And since the capacity of the digestive 
apparatus remains the same, the absorption of fat-producing 
materials in excess, implies defect in the absorption of ma- 
terials from which the tissues are formed, and out of which 
young ones are built up. Moreover, this special 
feeding with a view to rapid and early fattening, continued 
as it is through generations, and accompanied as it is by 
a selection of individuals and varieties which fatten most 
readily, tends to establish a modified constitution, more fitted 
for producing fat and correspondingly-less fitted for producing 
flesh—a constitution which, from this relatively-deficient ab- 
sorption of nitrogenous matters, is likely to become infertile ; 
as, indeed, these varieties generally become. Hence, 
no conclusions respecting the effects of high nutrition, pro- 
perly so called, can be drawn from cases of this kind. The 
cases are, in truth, of a kind that could not exist but for 
human agency. Under natural conditions no animal would 
diet itself in the way required to produce such results. And 
if it did, its race would quickly disappear.* 


* It is worth while inquiring whether unfitness of the food given to them, is 
not the chief cause of that sterility which, as Mr. Darwin says, ‘‘ is the great 
bar to the domestication of animals.” He remarks that ‘‘ when animals and 
plants are removed from their natural conditions, they are extremely liable to 


462 LAWS OF MULTIPLICATION. 


There is yet another mode in which accumulation of fat 
diminishes fertility. ven supposing it unaccompanied by 
a smaller absorption of nitrogenous materials, it is still a 
cause of lessening the surplus of nitrogenous materials. For 
the repair of the motor tissues becomes more costly. Fat 
stored-up is weight to be carried. A creature loaded with 
inert matter must, other things equal, consume a greater 
amount of tissue-forming substances for keeping its loco- 
motive apparatus in order ; and thus expending more for self- 
maintenance can expend less for race-maintenance. Abnormal 
plethora is thus antagonistic to reproduction in a double way. 
It ordinarily implies a smaller absorption of tissue-forming 
matters, and an increased demand on the diminished supply. 
Hence fertility decreases in a geometrical progression. 

The counter-conclusion drawn from facts of this class, is, 
then, due to a misconception of their nature—a misconception 
arising partly from the circumstance that the increase of bulk 
produced by fat is somewhat like the increase of bulk which 
erowth of tissues causes, and partly from the circumstance 
that abundance of good food normally produces a certain 
quantity of fat, which, within narrow limits, is a valuable 
store of force-evolving material. When, however, we limit 
the phrase high nutrition to its proper meaning—an abun- 
dance of, and due proportion among, all the substances which 
the organism needs—we find that, other things equal, fertility 
always increases as nutrition increases. And we see that these 
apparently-exceptional cases, are cases that really show us the 
same thing; since they are cases of relative innutrition. 


have their reproductive systems seriously affected.” Possibly the relative or 
absolute arrest of genesis, is less due to a direct effect on the reproductive sys- 
tem, than to a changed nutrition of which the reproductive system most clearly 
shows the results. The matters required for forming an embryo are ina 
greater proportion nitrogenous than are the matters required for maintain- 
ing an adult. Hence, an animal forced to live on insufficiently-nitrogenized 
‘ food, may have its surplus for reproduction cut off, but still have a sufficiency 
to keep its own tissues in repair, and appear to be in good health—meanwhile 
increasing in bulk from excess of the non-nitrogenous matters it eats. 


CHAPTER X. 
SPECIALITIES OF THESE RELATIONS. 


§ 356. Tests of the general doctrines set forth in preceding 
chapters, are afforded by organisms having modes of life that 
diverge widely from ordinary modes. Here, as elsewhere, 
aberrant cases yield crucial proofs. 

If certain organisms are so circumstanced that highly- 
nutritive matter is supplied to them without stint, and they 
have nothing to do but absorb it, we may infer that their 
powers of propagation will be enormous. 

If there are classes of creatures that expend very little for 
self-support in comparison with allied creatures, a relatively 
extreme prolificness may be expected of them. 

Or if, again, we find species presenting the peculiarity 
that while some of their individuals have much to do and 
little to eat, others of their individuals have much to eat and 
little to do, we may look for great fertility in these last and 
comparative infertility or barrenness in the first. 

These several anticipations we shall find completely 
verified. 


§ 357. Plants which, like the Rafflesiacee, carry their para- 
sitism to the extent of living on the juices they absorb from 
other plants, exhibit one of these relations in the vegetal 
kingdom. In them the organs for self-support being need- 
less, are rudimentary; and the parts directly or indirectly 


464 LAWS OF MULTIPLICATION. 


concerned in the production and distribution of germs, con- 
stitute the mass of the organism. That small ratio which 
the race-preserving structures bear to the self-preserving 
structures in ordinary Phenogams, is, in these Phzenogams, 
inverted. A like relation occurs in the common Dodder. 

There may be added a kindred piece of evidence which the 
Fungi present. Those of them which grow on living plants, 
repeat the above connection completely ; and those of them 
which, though not parasitic, nevertheless subsist on organized 
materials previously elaborated by other plants, substantially 
repeat it. The spore-producing part is relatively enormous ; 
and the fertility is far greater than that of Cryptogams of like 
sizes, which have to form for themselves the organic com- 
pounds of which they and their germs consist. 

§ 358. The same lesson is taught us by animal-parasites. 
Along with the decreased cost of Individuation, they similarly 
show us an increased expenditure for Genesis; and they show 
us this in the most striking manner where the deviation from 
ordinary conditions of life is the greatest. 

Take, among the Epizoa, such an instance as the Wicothe. 
Belonging to the Hntomostraca, both males and females of 
this species are, in their early days, similar to their allies; 
and the males continue so throughout life. Each female, 
however, presently fixes herself on the skin of an aquatic 
animal, where she sits and sucks its juices, enlarges rapidly, 
and undergoes an extreme distortion from the growth of 
the ovaries. These, bulging out from her sides, become lateral 
sacs, each of which attains somethin, like three times her 
size; and then a further distortion is produced by two vast 
ege-bags, severally larger than herself, which also are formed 
and become pendant. So that the germ-producing organs and 
their contents, eventually acquire a total bulk some eight or 
ten times that of the rest of the body. Numerous species of 
this type and habit, repeat this relation between a life of in- 
action with high feeding, and an enormous rate of - genesis, | 


SPECIALITIES OF TRESE RELATIONS. 465 


Entozoa yield us many examples of this causal relation, 
raised to a still higher degree. The Gordius, or Hair-worm, 
is a creature which, finding its way when young into the 
body of an insect, there grows rapidly, and afterwards emerg- 
ing to breed, lays as many as 8,000,000 eggs in less than a day. 
Similarly with the larger types that infest the higher 
animals. It has been calculated by Dr. Eschricht, as quoted 
by Professor Owen, that there are “64,000,000 of ova in tha 
mature female Ascaris Lumbricoides.”’ ven a still greater 
fertility occurs among the cestoid Hntozoa. Immersed as a 
Tape-worm is in nutritive liquid, which it absorbs through its 
integument, it requires no digestive apparatus. The room 
which one would occupy, and the materials it would use up, 
are therefore available for germ-producing organs, which 
nearly fill each segment: each segment, sexually complete in 
itself, is little else than an enormous reproductive system, 
with just enough of other structures to bind it together. 
Remembering that the Tape-worm, retaining its hold, con- 
tinues to bud-out such segments as fast as the fully-developed 
ones are cast off, and goes on doing this as long as the infested 
individual lives; we see that here, where there is no ex- 
penditure, where the cost of individuation is reduced to the 
greatest extent while the nutrition is the highest possible, 
the degree of fertility reaches its extreme. These 
dintozoa yield us further interesting evidence. Of their 
various species, most if’not all undergo passive migration from 
animal to animal before they become nature. Usually, the 
form assumed in the body of the first host, is devoid of all 
that part in which the reproductive structures take their rise ; 
and this part grows and develops reproductive structures, 
only in some predatory animal to which its first host falls a 
sacrifice. Occasionally, however, the egg gives origin to the 
sexual form in the animal that originally swallowed it, but 
the development remains incomplete—there is no sexual 
genesis, no formation of eggs in the rudimentary segments. 
That these may become fertile, it is needful, as before, for the 

VOL. Il. 30 


466 LAWS OF MULTIPLICATION. 


containing animal to be devoured ; so that the imperfect Tape- 
worm may find its way into the intestine of a higher animal. 
Thus the Bothriocephalus solidus, found in the abdominal cavity 
of the Stickleback, is barren while it remains there ; but if the 
Stickleback is eaten by a Water-fowl, the reproductive system 
of the transferred Bothriocephalus becomes developed and 
active. So, too, a kind of Tape-worm which remains infertile 
while in the intestine of a Mouse, becomes fertile in the in- 
testine of a Cat that devours the mouse. May we not regard 
these facts as again showing the dependence of fertility on 
nutrition ? Barrenness here accompanies conditions unfavour- 
able to the absorption of nutriment; and it gives way to 
fecundity where nutriment is large in quantity and superior 
in quality. 


§ 359. Extremely significant are those cases of partial 
reversion to primitive forms of genesis, that occur under 
special conditions in some of the higher Annulosa. I refer to 
the pseudo-parthenogenesis and metagenesis in Insects. 

Under what conditions do the Aphides exhibit this strange 
deviation from the habits of their order? Why among them 
should imperfect females produce, agamically, others like 
themselves, generation after generation, with great rapidity ? 
There is the obvious explanation that they get plenty of 
easily-assimilated food without exertion. Piercing the tender 
coats of young shoots, they sit and suck—appropriating the 
nitrogenous elements of the sap and ejecting its saccharine 
matter as “honey dew.” Along with a sluggishness 
strongly contrasted with the activity of their allies—along 
with a very low rate of consumption and a correlative degra- 
dation of structure; we have here a retrogression to asexual 
genesis, and a greatly-increased rate of multiplication. 

The recently-discovered instance of internal metagenesis 
in the maggots of certain Flies has a like meaning. In- 
credible as it at first seemed to naturalists, it is now proved that 
the Cecydomia-larva develops in its interior a brood of larvze 


SPECIALITIES OF THESE RELATIONS. 467 


of like structure with itself. In this case, as in the last, abun- 
dant food is combined with low expenditure. These larvee are 
found in such habitats as the refuse of beet-root-sugar fac- 
tories—masses of nitrogenous débris remaining after the 
extraction of the saccharine matter. Hach larva has a 
practically-unlimited supply of sustenance imbedding it on 
all sides. 

It is true that some other maggots, as those of the Flesh-fly, 
are similarly, or still better, cireumstanced; and, it may be 
said, ought therefore to have the same habit. But this does 
not necessarily follow. Survival of the fittest will determine 
whether such specially-favourable conditions result in the 
agerandisement of the individual or in the multiplication of 
the race. And in the case of the Flesh-fly, there is a reason 
why greater individuation rather than more rapid genesis 
will occur. For a decomposing animal body lasts so short a 
time, that were Flesh-fly larve to multiply agamically, the 
second generation would die from the disappearauce of their 
food. Hence, individuals in which the excessive nutrition 
led to internal metagenesis, would leave no posterity; and 
natural selection would establish the variety in which greater 
growth resulted. All which the argument requires is, that 
when such reversion to agamogenesis does take place, it shall 
be where the food is unusually abundant and the expenditure 
unusually small; and this the cases instanced go to show. 


§ 360. The physiological lesson taught us by Bees and 
Ants, not quite harmonizing with the moral lesson they are 
supposed to teach, is that highly-fed idleness is favourable to 
fertility, and that excessive industry has barrenness for its 
concomitant. 

The egg of a Bee develops into a small barren female or 
into a large fertile female, according to the supply of food 
given to the larva hatched from it. We here see that the 
germ-producing action is an overflow of the surplus remain- 


ing after completion of the individual; and that the lower 
30 * 


468 LAWS OF MULTIPLICATION. 


feeding which the larva of a working Bee has, results in a 
dwarfing of the adult and an arrested development of the 
generative organs. Further, we have the fact that the con- 
dition under which the perfect female, or mother-Bee, goes 
on, unlike insects in. general, laying eggs continuously, is 
that she has plenty of food brought to her, is kept warm, and 
goes through no considerable exertion. While, contrariwise, 
it is to be noted that the infertility of the workers, is asso- 
ciated with the ceaseless labour of bringing materials for the 
combs and building them, as well as the labour of feeding 
the queen, the larvee, and themselves. 

Ants, and especially some of the tropical kinds, show 
us these relations in an exaggerated form. The differ- 
ence of bulk between the fecund and infecund females is 
immensely greater. The mother-Ant has the reproductive 
system so enormously developed, that the remainder of her 
body is relatively insignificant. Entirely incapable of loco- 
motion, she is unable to deposit her eggs in the places where 
they are to be hatched; so that they have to be carried away 
by the workers as fast as they are extruded. Her life is thus 
reduced substantially to that of a parasite—an absorption of 
abundant food supplied gratis, a total absence of expendi- 
ture, and a consequent excessive rate of genesis. ‘ The 
queen-ant of the African Termites lays 80,000 eggs in twenty- 
four hours.” 


§ 361. It may be needful to say that these exceptional 
relations cannot be ascribed to the assigned causes acting 
alone. The extreme fertility which, among parasites and 
social insects, accompanies extremely high feeding, and an 
expenditure reduced nearly to zero, presupposes typical struc- 
tures and tendencies of suitable kinds; and these are not 
directly accounted for. On creatures otherwise organized, 
unlimited supplies of food and total inactivity are not fol- 
lowed by such results. There of course requires a consti- 
tution fitted to the special conditions; and the evolution of 


SPECIALITIES OF THESE RELATIONS. 469 


this cannot be due simply to plethora joined with rest. These 
cases are given as illustrating the conditions under which 
extreme exaltations of fertility become possible. Their mean- 
ings, thus limited, are clear, and completely to the point. We 
see in them that the devotion of nutriment to race-preserva- 
tion, is carried furthest where the cost of self-preservation 
is reduced to a minimum; and, conversely, that nothing 
is devoted directly to race-preservation by individuals on 
which falls an excessive expenditure for self-preservation and 
preservation of other’s offspring. 


CHAPTER XI. 
INTERPRETATION AND QUALIFICATION. 


§ 362. Considering the difficulties of inductive verification, 
we have, I think, as clear a correspondence between the 
d priori and a posteriori conclusions, as can be expected. The 
many factors co-operating to bring about the result in every 
case, are so variable in their absolute and relative amounts, 
that we can rarely disentangle the effect of each one; and 
have usually to be content with qualified inferences. Though 
in the mass, organisms show us an unmistakable relation 
between great size andsmall fertility ; yet special comparisons 
among them are nearly always partially vitiated by differ- 
ences of structure, differences of nutrition, differences of 
expenditure. Though it is beyond question that the more 
complex organisms are the less prolific; yet as complexity has 
a certain general connexion with bulk, and in animals with 
expenditure, we cannot often identify its results as inde- 
pendent of these. And, similarly, though the creatures that 
waste much matter in producing motion, sensible and insen- 
sible, have lower rates of multiplication than those which 
waste less; yet, as the creatures which waste much are 
generally larger and more complex, we are again met by an 
obstacle which limits our comparisons, and compels us to 
accept conclusions less definite than are desirable. 

Such difficulties arise, however, only when we endeavour, 
as in foregoing chapters, to prove the inverse variation 


INTERPRETATION AND QUALIFICATION. 471 


between Genesis and each separate element of Individuation 
—growth, development, activity. We are scarcely at all 
hampered by qualifications when, from contemplating these 
special relations, we return to the general relation. The 
antagonism between Individuation and Genesis, is shown by 
all the facts that have been grouped under each head. We 
have seen that in ascending from the lowest to the highest 
types, there is a decrease of fertility so great as to be abso- 
lutely inconceivable, and even inexpressible by figures; and 
whether the superiority of type consists in relative largeness, 
in greater complexity, in higher activity, or in some or all of 
these combined, matters not to the ultimate inference. The 
broad fact, enough for us here, is that organisms in which 
the integration and differentiation of matter and motion have 
been carried furthest, are those in which the rate of multipli- 
cation has fallen lowest. How much of the decline of repro- 
ductive power is due to the greater integration of matter, 
how much to its greater differentiation, how much to the 
larger amounts of integrated and differentiated motions gene- 
rated, it may be impossible to say; and it is not needful to 
say. These are all elements of a higher degree of life, an 
augmented ability to maintain the organic equilibrium amid 
environing actions—an increased power of self-preservation ; 
and we find their invariable accompaniment to be, a dimi- 
nished expenditure of matter, or motion, or both, in race- 
preservation. 

In brief, then, examination of the evidence shows that 
there does exist that relation which we inferred mus¢ exist. 
Arguing from general data, we saw that for the maintenance 
of a species, the ability to produce offspring must be great, 
in proportion as the ability of the individuals to contend with 
destroying forces is small; and conversely. Arguing from 
other general data, we saw that, derived as the self-sustain- 
ing and race-sustaining forces are from a common stock of 
force, it necessarily happens that, other things equal, increase 
of one involves decrease of the other. And then, turning 


472 LAWS OF MULTIPLICATION. 


to special facts, we have found that this inverse variation is 
clearly traceable throughout both the animal and vegetal 
kingdoms. We may therefore set it down as a law, that 
every higher degree of organic evolution, has for its con- 
comitant a lower degree of that peculiar organic dissolution 
which is seen in the production of new. organisms. 


§ 363. Something remains to be said in reply to the in- 
quiry—how is the ratio between Individuation and Genesis 
established in each case? This inquiry has been but partially 
answered in the course of the foregoing argument. 

All specialities of the reproductive process are due to the 
natural selection of favourable variations. Whether a creature 
lays a few large eggs or many small ones equal in weight to 
the few large, is not determined by any physiological neces- 
sity : here the only assignable cause is the survival of varieties 
in which the matter devoted to reproduction, happens to be 
divided into portions of such size and number as most to 
favour multiplication. Whether in any case there are 
frequent small broods or larger broods at longer intervals, 
depends wholly on the constitutional peculiarity that has 
arisen from the dying out of families in which the sizes and 
intervals of the broods were least suited to the conditions of 
life. Whether a species of animal produces many offspring 
of which it takes no care or a few of which it takes much 
care—that is, whether its reproductive surplus is laid out 
wholly in germs or partly in germs and partly in labour on 
their behalf—must have been decided by that moulding of 
constitution to conditions, slowly effected through the more 
frequent preservation of descendants from those whose re- 
productive habits were best adapted to the circum- 
stances of the species. Given a certain surplus available 
for race-preservation, and it is clear that by indirect 
equilibration only, can there be established the more or 
less peculiar distribution of this surplus which we see in 
each case. Obviously, too, survival of the fittest 


INTERPRETATION AND QUALIFICATION. 473 


has a share in determining the proportion between the 
amount of matter that goes to Individuation and the amount 
that goes to Genesis. Whether the interests of the species 
are most subserved by a higher evolution of the individual 
joined with a diminished fertility, or by a lower evolution of 
the individual joined with an increased fertility, are ques- 
tions ever being experimentally answered. If the more- 
developed and less-prolific variety has a greater number of 
survivors, it becomes established and predominant. If, con- 
trariwise, the conditions of life being simple, the larger or 
more-organized individuals gain nothing by their greater size 
or better organization ; then the greater fertility of the less 
evolved ones, will insure to their descendants an increasing 
predominance. 

But direct equilibration all along maintains the limits 
within which indirect equilibration thus works. The 
necessary antagonism we have traced, rigidly restricts the 
changes that natural selection can produce, under given con- 
ditions, in either direction. A greater demand for Individua- 
tion, be it a demand caused by some spontaneous variation or 
by an adaptive increase of structure and function,. inevitably 
diminishes the supply for Genesis; and natural selection 
cannot, other things remaining the same, restore the rate of 
Genesis while the higher Individuation is maintained. Con- 
versely, survival of the fittest, acting on a species that has, 
by spontaneous variation or otherwise, become more prolific, 
cannot again raise its lowered Individuation, so long as every- 
thing else continues constant. 


§ 364. Here, however, a qualification must be made. It 
was parenthetically remarked in § 327 that the inverse varia- 
tion between Individuation and Genesis is not exact; and it 
was hinted that a slight modification of statement would be 
requisite at a more advanced stage of the argument. We 
have now reached the proper place for specifying this 
modification. 


474 ‘LAWS OF MULTIPLICATION. 


Each increment of evolution entails a decrement of re- 
production that is not accurately proportionate, but somewhat 
less than proportionate. The gain in the one direction is not 
wholly canceled by a loss in the other direction, but only 
partially canceled : leaving a margin of profit to the species. 
Though augmented power of self-maintenance habitually 
necessitates diminished power of race-propagation, yet the 
product of the two factors is greater than before ; so that the 
forces preservative of race become, thereafter, in excess of the 
forces destructive of race, and the race spreads. We shall 
soon see why this happens. 

Each advance in evolution implies an economy. That any 
increase in bulk, or structure, or activity, may become esta- 
blished, the life of the organism must be to some extent 
facilitated by the change—the cost of self-support must be, 
on the average, reduced. If the greater complexity, or the 
larger size, or the more agile movement, entails on the in- 
dividual an outlay that is not repaid in food more-easily 
obtained, or danger more-easily escaped ; then the individual 
will be at a relative disadvantage, and its diminished posterity 
will disappear. If the extra outlay is but just made good 
by the extra advantage, the modified individual will not sur- 
vive longer, or leave more descendants, than the unmodified 
individuals. Consequently, it is only when the expense of 
greater individuation is out-balanced by a subsequent saving, 
that it can tend to subserve the preservation of the indi- 
vidual; or, by implication, the preservation of the race. 
The vital capital invested in the alteration must bring 
a more than equivalent return. A few instances 
will show that, whether the change results from direct 
equilibration or from indirect equilibration, this must happen. 
Suppose a creature takes to performing some act in an un- 
usual way—leaps where ordinarily its kindred crawl, eludes 
pursuit by diving instead of, like others of its kind, by swim- 
ming along the surface, escapes by doubling instead of by sheer 
speed. Clearly, perseverance in the modified habit will, other 


INTERPRETATION AND QUALIFICATION. 475 


things equal, imply that it takes less effort. The creature’s 
sensations will ever prompt desistance from the more laborious 
course; and hence a congenital habit is not likely to be 
diverged from unless an economy of force is achieved by the 
divergence. Assuming, then, that the new method has no 
advantage over the old in directly diminishing the chances 
of death, the establishment of it, and of the structural 
complications involved, nevertheless implies a physiological 
gain. Suppose, again, that an animal takes to some 
abundant food previously refused by its kind. It is likely to 
persist only if that the comparative ease in obtaining this 
food, more than compensates for any want of adaptation to its 
digestive organs; so that superposed modifications of the 
digestive organs are likely to arise only when an average 
economy results. What now must be the influence 
on the creature’s system as a whole? Diminished expenditure 
in any direction, or increased nutrition however effected, 
will leave a greater surplus of materials. The animal will be 
physiologically richer. Part of its augmented wealth will go 
towards its own greater individuation—its size, or its strength, 
or both, will increase; while another part will go towards 
more active genesis. Just as a state of plethora directly 
produced enhances fertility; so will such a state indirectly 
produced. 

In another way, the same thing must result from those 
additions to bulk or complexity or activity that are due to 
survival of the fittest. Any change which prolongs individual 
life, will, other things remaining the same, further the pro- 
duction of offspring. KEven when it is not, like the foregoing, 
a means of economizing the forces of the individual, still, if it 
increases the chances of escaping destruction, it increases the 
chances of leaving posterity. Any further degree of evolution, 
therefore, will be so established only where the cost of it is 
more than repaid; part of the gain being shown in the 
lengthened life of the individual, and part in the greatey 
proauction of other individuals. 


476 LAWS OF MULTIPLICATION. 


We have here the solution of various minor anomalies by 
which the inverse variation of Individuation and Genesis is 
obscured. ‘Take as an instance the fertility of the Blackbird 
as compared with that of the Linnet. Both birds lay five eggs, 
and both usually have two broods. Yet the Blackbird is far 
the larger of the two; and ought, according to the general 
law, to be much less prolific. What causes this noncon- 
formity ? We shall find an answer in their respective foods 
and habits. Except during the time that it is rearing its 
young, the Linnet collects only vegetal food—lives during 
the winter on the seeds it finds in the fields, or, when hard 
pressed, picks up around farms; and to obtain this spare 
diet is continually flying about. The result, if it survives the 
frost and snow, is a considerable depletion; and it recovers 
its condition only after some length of spring weather. The 
Blackbird, on the other hand, is omnivorous: while it eats 
grain and fruit when they come in its way, 1t depends largely 
on animal food. It cuts to pieces and devours the dew-worms 
which, morning and evening, it finds on the surface of a lawn, 
and, even discovering where they are, unearths them; it 
swallows slugs, and breaking snail-shells, either with its beak 
or by hammering them against stones, tears out their tenants; 
and it eats beetles and larve. Thus the strength of the 
Blackbird opens to it a store of good food, much of which is 
inaccessible to so small and weak a bird as a Linnet—a store 
especially helpful to it during the cold months, when the 
hybernating Snails in hedge-bottoms yield it abundant pro- 
vision. The result is that the Blackbird is ready to breed 
very early in spring; and is able during the summer to rear 
a second, and sometimes even a third, brood. Here, then, a 
higher degree of Individuation secures advantages so great, 
as to much more than compensate its cost: it is not that the 
decline of Genesis is less than proportionate to the increase of 
Individuation, but there is no decline at all. Com- 
parison of the Rat with the Mouse yields a parallel result. 
Though they differ greatly in size, yet the one is as prolific 


IN TERPRETATION AND QUALIFICATION. 477 


as the other. This absence of difference cannot be ascribed 
to their unlike degrees of activity. We must seek its cause in 
some facility of living secured to the Rat by its greater intel- 
ligence, greater power and courage, greater ability to utilize 
vhat it finds. The Rat is notoriously cunning; and its 
cunning gives success to its foraging expeditions. It is not, 
like the Mouse, limited mainly to vegetal food; but while it 
eats grain and beans like the Mouse, it also eats flesh and 
carrion, devours young poultry and eggs. The result is that, 
without a proportionate increase of expenditure, it gets a far 
larger supply of nourishment than the Mouse ; and this rela- 
tive excess of nourishment makes possible a large size without 
a smaller rate of multiplication. How clearly this is the 
cause, we see in the contrast between the common Rat and 
the Water-Rat. While the common Rat has habitually 
several broods a-year of from 10 to i2 each, the Water-Rat, 
though somewhat smaller, has but 5 or 6 in a brood, and but 
one brood, or sometimes two broods, a-year. But the Water- 
Rat lives on vegetal food—lacks all that its bold, sagacious, 
omnivorous congener, gains from the warmth as well as the 
abundance which men’s habitations yield. 

The inverse variation of Individuation and Genesis is, 
therefore, but approximate. Recognizing the truth that 
every increment of evolution which is appropriate to the 
circumstances of an organism, brings an advantage somewhat 
in excess of its cost; we see the general law, as more strictly 
stated, to be that Genesis decreases not quite so fast as In- 
dividuation increases. Whether the greater Individuation 
takes the form of a larger bulk and accompanying access of 
strength ; whether it be shown in higher speed or agility ; 
whether it consists in a modification of structure that 
facilitates some habitual movement, or in a visceral change 
that helps to utilize better the absorbed aliment; the ultimate 
effect is identical. ‘There is either a more economical per- 
formance of the same actions, internal or external, or there 
is a securing of greater advantages by modified actions, which 


478 LAWS OF MULTIPLICATION. 


cost no more, or have an increased cost less than the in- 

creased gain. In any case, the result is a greater surplus of 
vital capital; part of which goes to the aggrandisement 
of the individual, and part to the formation of new in- 
dividuals. While the higher tide of nutritive matters, 
everywhere filling the parent-organism, adds to its power of 
self-maintenance, it also causes a reproductive overflow larger 
than before. 

Hence every type that is best adapted to its conditions, 
which on the average means every higher type, has a rate of 
multiplication that insures a tendency to predominate. 
Survival of the fittest, acting alone, is ever replacing in- 
ferior species by superior species. But beyond the longer 
survival, and therefore greater chance of leaving offspring, 
which superiority gives, we see here another way in which 
the spread of the superior is insured. Though the more- 
evolved organism is the less fertile absolutely, it is the more 
fertile relatively. 


CHAPTER XII. 
MULTIPLICATION OF THE HUMAN RACE. 


§ 365. The relative fertility of Man considered as a species, 
ind those changes in Man’s fertility which occur under 
changed conditions, must conform to the laws which we have 
traced thus far. As a matter of course, the inverse variation 
between Individuation and Genesis, holds of him as of all 
other organized beings. His extremely low rate of multipli- 
cation—far below that of all terrestrial Mammals except the 
Elephant, (which though otherwise less evolved, is, in extent 
of integration, more evolved)—we shall recognize as the 
necessary concomitant of his much higher evolution. And 
the causes of increase or decrease in his fertility, special or 
general, temporary or permanent, we shall expect to find in 
those changes of bulk, of structure, or of expenditure, which 
we have in all other cases seen associated with such effects. 

In the absence of detailed proof that these parallelisms 
exist, it might suffice to contemplate the several communities 
between the reproductive function in human beings and other 
beings. Ido not refer simply to the fact that genesis pro- 
ceeds in a similar manner; but I refer to the similarity of 
the relation between the generative function and the func- 
tions that have for their joint end the preservation of the 
individual. In Man, as in other creatures that expend much. 
genesis commences only when growth and development are 
declining in rapidity and approaching their termination. 
Among the higher organisms in general, the reproductive 


480 LAWS OF MULTIPLICATION. 


activity, continuing during the prime of life, ceases when the 
vigour declines, leaving a closing period of infertility; and in 
like manner among ourselves, barrenness supervenes when 
middle age brings the surplus vitality to an end. So, too, 
it is found that in Man, as in beings of lower orders, there is 
a period at which fecundity culminates. In § 341, facts were 
cited showing that at the commencement of the reproductive 
period, animals bear fewer offspring than afterwards; and 
that towards the close of the reproductive period, there is a 
decrease in the number produced. In like manner it is shown 
by the tables of Dr. Duncan’s recent work, that the fecundity 
of women increases up to the age of about 25 years; and 
continuing high with but slight diminution till after 30, 
then gradually wanes. It is the same with the sizes and 
weights of offspring. Infants born of women from 25 to 29 
years of age, are both longer and heavier than infants born 
of younger or older women; and this difference has the same 
implication as the greater total weight of the offspring pro- 
duced at a birth, during the most fecund age of a pluriparous 
animal. Once more, there is the fact that a too-early bearing 
of young produces on a woman the same injurious effects as 
on an inferior creature—an arrest of growth and an enfeeble- 
ment of constitution. — 

Considering these general and speciai parallelisms, we 
might safely infer that variations of human fertility conform 
to the same laws as do variations of fertility in general. 
But it is not needful to content ourselves with an implication. 
Evidence is assignable that what causes increase or decrease 
of genesis in other creatures, causes increase or decrease of 
genesis in Man. It is true that, even more than hitherto, our 
reasonings are beset by difficulties. So numerous are the 
inequalities in the conditions, that but few unobjectionable 
comparisons can be made. ‘The human races differ consider- 
ably in their sizes, and notably in their degrees of cerebral 
development. The climates they inhabit entail on them 
widely different consumptions of matter for maintenance ot 


MULTIPLICATION OF THE HUMAN RACE. 481 


temperature. Both in their qualities ana quantities, the 
foods they live on are unlike ; and the supply is here regular 
and there very irregular. Their expenditures in bodily action 
are extremely unequal; and even still more unequai are 
their expenditures in mental action. Hence the factors, 
varying so much in their amounts and combinations, can 
scarcely ever have their respective effects identified. Never- 
theless there are a few comparisons, the results of which may 
withstand criticism. 


§ 366. The increase of fertility caused by a nutrition that 
is greatly in excess of the expenditure, is to be detected by 
contrasting populations of the same race, or allied races, 
one of which obtains good and abundant sustenance much 
more easily than the other. ‘Three cases may here be set 
down. 

The traveller Barrow, describing the Cape-Boors, says :— 
“ Unwilling to work and unable to think,” * * * “indulging 
to excess in the gratification of every sensual appetite, the 
African peasant grows to an unwieldy size;” and respecting 
the other sex, he adds—“ the women of the African peasantry 
lead a life of the most listless inactivity.’ Then, after illus- 
trating these statements, he goes on to note “the prolific 
tendency of all the African peasantry. Six or seven children 
in a family are considered as very few; from a dozen to 
twenty are not uncommon.” The native races of 
this region yield evidence to the same effect. Speaking of 
the cruelly-used Hottentots (he’is writing sixty years ago), 
who, while they are poor and ill-fed, have to do all the work 
for the idle Boors, Barrow says that they “seldom have more 
than two or three children; and many of the women are 
barren.” This unusual infertility stands in remarkable con- 
trast with the unusual fertility of the Kaffirs, of whom he 
afterwards gives an account. Rich in cattle, leading easy 
lives, and living almost exclusively on animal food (chiefly 


milk with occasional flesh), these people were then reputed 
VoL. It 31 


482 LAWS OF MULTIPLICATION 


to have a very high rate of multiplication. Barrow writes :— 
“They are said to be exceedingly prolific; that twins are 
almost as frequent as single births, and that it is no un- 
common thing for a woman to have three at a time.” Pro- 
bably both these statements are in excess of the truth; but 
there is room for large discounts without destroying the 
extreme difference. A third instance is that of the 
French Canadians. ‘‘ Nous sommes terribles pour les enfants !” 
observed one of them to Prof. Johnston; who tells us that 
the man who said this ‘‘ was one of fourteen children—was 
himself the father of fourteen, and assured me that from 
eight to sixteen was the usual number of the farmers’ 
families. He even named one or two women who had 
brought their husbands five-and-twenty, and threatened ‘ de 
vingt-sixiéme pour le prétre.” From these large families, 
joined with the early marriages and low rate of mortality, it 
results that, by natural increase, “there are added to the 
French-Canadian population of Lower Canada four persons. 
for every one that 1s added to the population of England.” 
Now these French-Canadians are described by Prof. Johnston 
as home-loving, contented, unenterprising; and as living in 
a region where “land and subsistence are easily obtained.” 
Very. moderate industry brings to them liberal supplies of 
necessaries ; and they pass a considerable portion of the year 
in idleness. Hence the cost of Individuation being much 
reduced, the rate of Genesis is much increased. That this 
uncommon fertility is not due to any direct influence of the 
locality, is implied by the fact that along with the “ restless, 
discontented, striving, burning energy of their Saxon neigh- 
bours” no such rate of multiplication is observed; while 
further south, where the physical circumstances are more 
favourable if anything, the Anglo-Saxons, leading lives of 
excessive activity, have a fertility below the average. And 
that the peculiarity is not a direct effect of race, is proved by 
the fact that in Europe, the rural French are certainly not 
more prolific than the rural English. 


MULTIPLICATION OF THE HUMAN RACE. 483 


To every reader there will probably occur the seemingly- 
adverse evidence furnished by the Irish; who, though not 
well fed, multiply fast. Part of this more rapid increase is 
due to the earlier marriages common among them, and con- 
sequent quicker succession of generations—a factor which, 
as we have seen, has a larger effect than any other on the 
rate of multiplication. Part of it is due to the greater 
generality of marriage—to the comparative smallness of the 
number who die without having had the opportunity of pro- 
ducing offspring. The effects of these causes having been 
deducted, we may doubt whether the Irish, individually con- 
sidered, would be found more prolific than the English. 
Perhaps, however, it will be said that, considering their diet, 
they ought to be less prolific. This is by no means obvious. 
It is not simply a question of nutriment absorbed: it is a 
question of how much remains after the expenditure in self- 
maintenance. Now a notorious peculiarity in the life of the 
Trish peasant, is, that he obtains a return of food that is large 
in proportion to his outlay in labour. The cultivation of his 
potatoe-ground occupies each cottager but a small part of the 
year; and the domestic economy of his wife is not of a kind 
to entail on her much daily exertion. Consequently, the crop, 
tolerably abundant in quantity though innutritive in quality, 
very possibly suffices to meet the comparatively-low expendi- 
ture, and to leave a good surplus for genesis—perhaps a 
greater surplus than remains to the males and females of the 
English peasantry, who, though fed on better food, are 
harder worked. 

We conclude, then, that in the human race, as in all other 
races, such absolute or relative abundance of nutriment as 
leaves a large excess after defraying the cost of carrying on 
parental life, is accompanied by a high rate of genesis.* 


* This is exactly the reverse of Mr. Doubleday’s doctrine; which is that 
throughout both the animal and vegetal kingdoms, ‘‘ over-feeding checks in- 
crease ; whilst, on the other hand, a limited or deficient nutriment stimu- 
lates and adds to it.” Or, as he elsewhere says—‘‘ Be the range of the 

Fle 


484 LAWS OF MULTIPLICATION. 


§ 367. Evidence of the converse truth, that relative in- 
crease of expenditure, leaving a diminished surplus, reduces 
the degree of fertility, is not wanting. Some of it has been 
set down for the sake of antithesis in the foregoing section. 
Here may be grouped a few facts of a more special kind 
having the same implication. 

To prove that much bodily labour renders women less 
prolific, requires more evidence than is obtainable. Some evi- 
dence, however, may be set down. De Boismont in France and 
Dr. Szukits in Austria, have shown by extensive statistical 
comparisons, that the reproductive age is reached a year 
later by women of the labouring class than by middle-class 
women; and while ascribing this delay in part to inferior 


natural power to increase in any species what it may, the plethoric state 
invariably checks it, and the deplethoric state invariably develops it ; and this 
happens in the exact ratio of the intensity and completeness of each state, 
until each state be carried so far as to bring about the actual ueath of the 
animal or plant itself.” 

I have space here only to indicate the misinterpretations on which Mr. 
Doubleday has based his argument. 

In the first place, he has confounded normal plethora with what I have, in 
§ 355, distinguished as abnormal plethora. The cases of infertility accom- 
panying fatness, which he cites in proof that over-feeding checks increase, are 
not cases of high nutrition properly so called; but cases of such defective 
absorption or assimilation as constitutes low nutrition. In Chap. IX, abun- 
dant proof was given that a truly plethoric state is an unusually fertile state. 
It may be added that much of the evidence by which Mr. Doubleday seeks to 
show that among men, highly-fed classes are infertile classes, may be out- 
balanced by counter-evidence. Many years ago Mr. Lewes pointed this out : 
extracting from a book on the peerage, the names of 16 peers who had, at that 
time, 186 children ; giving an average of 11°6 in a family. 

Mr. Doubleday insists much on the support given to his theory by the 
barrenness of very luxuriant plants, and the fruitfulness produced in plants 
by depletion. Had he been aware that the change from barrenness to fruit- 
fulness in plants, is a change from agamogenesis to gamogenesis—had it been as 
well known at the time when he wrote as it is now, that a tree which goes on 
putting out sexless shoots, is so producing new individuals; and that when it 
begins to bear fruit, it simply begins to produce new individuals after another 
manner—he would have perceived that facts of this class do not tell in his 
avour. 

In the law which Mr. Doubleday alleges, he sees a guarantee for the main- 


MULTIPLICATION OF THE HUMAN RACE. 485 


nutrition, we may suspect that it is in part due to greater 
muscular expenditure. A kindred fact, admitting of a 
kindred interpretation, may be added. Though the com- 
paratively-low rate of increase in France is attributed to 
other causes, yet, very possibly, one of its causes is the greater 
proportion of hard work entailed on French women, by the 
excessive abstraction of men for non-productive occupations, 
military and civil. The higher rate of multiplication in 
England than in continental countries generally, is not im- 
probably furthered by the easier lives which English women 
lead. 

That absolute or relative infertility is generally pro- 
duced in women by mental labour carried to excess, is more 
clearly shown. Though the regimen of upper-class girls is 
not what it should be, yet, considering that their feeding is 


tenance of species. He argues that the plethoric state of the individuals con- 
stituting any race of organisms, presupposes conditions so favourable to life 
that the race can be in no danger; and that rapidity of multiplication becomes 
needless. Conversely, he argues that a deplethoric state implies unfavourable 
conditions—implies, consequently, unusual mortality; that is—implies a 
necessity for increased fertility to prevent the race from dying out. It may 
be readily shown, however, that such an arrangement would be the reverse of 
self-adjusting. Suppose a species, too numerous for its food, to be in the 
resulting deplethoric state. It will, according to Mr. Doubleday, become 
unusually fertile ; and the next generation will be more numerous rather than 
less numerous. For, by the hypothesis, the unusual fertility due to the 
deplethoric state, is the cause of undue increase of population. Bnt if the 
next generation is more numerous while the supply of food has remained 
the same, or rather has decreased under the keener competition for it, 
then this next generation will be in-a still more deplethoric state, and 
will be still more fertile. Thus there will go on an ever-increasing rate 
of multiplication, and an ever-decreasing supply of food, until the species 
disappears. Suppose, on the other hand, the members of a species to be in 
an unusually plethoric state. Their rate of multiplication, ordinarily suffi- 
cient 10 maintain their numbers, will become insufficient to maintain their 
numbers. In the next generation, therefore, there will be fewer to eat the 
already abundant food, which, becoming relatively still more abundant, will 
render the fewer members of the species still more plethoric, and still less 
fertile, than their parents. And the actions and reactions continuing, the 
species will presently die out from absolute barrenness. 


486 LAWS OF MULTIPLICATION. 


better than that of girls belonging to the poorer classes, 
while, in most other respects, their physical treatment is not 
worse, the deficiency of reproductive power among them 
may be reasonably attributed to the overtaxing of their 
brains—an overtaxing which produces a serious reaction on 
the physique. This diminution of reproductive power is not 
shown only by the greater frequency of absolute sterility ; 
nor is it shown only in the earlier cessation of child-bearing ; 
but it is also shown in the very frequent inability of such 
women to suckle their infants. In its full sense, the re- 
productive power means the power to bear a well-developed 
infant, and to supply that infant with the natural food for 
the natural period. Most of the flat-chested girls who 
survive their high-pressure education, are incompetent to 
do this. Were their fertility measured by the number of 
children they could rear without artificial aid, they would 
prove relatively very infertile. 

The cost of reproduction to males being so much ee 
than it is to females, the antagonism between Genesis and 
Individuation is not often shown in men by suppression of 
generative power consequent on unusual expenditure in 
bodily action. Nevertheless, there are indications that this 
results in extreme cases. We read that the ancient athlete 
rarely had children ; and among such of their modern repre- 
sentatives as acrobats, an allied relation of cause and effect 
is alleged. Indirectly this truth, or rather its converse, 
appears to have been ascertained by those who train men for 
feats of strength—they find it needful to insist on con- 
tinence. 

Special proofs that in men, great cerebral expenditure di- 
minishes or destroys generative power, are difficult to obtain. 
It is, indeed, asserted that intense application to mathematics, 
requiring as it does extreme concentration of thought, is apt 
to have this result; and it is asserted, too, that this result is 
produced by the excessive emotional excitement of gambling. 
Then, again, it is a matter of common remark how frequently 


MULTIPLICATION OF THE HUMAN RACE. 487 


men of unusual mental activity leave no offspring. But 
facts of this kind admit of another interpretation. The re- 
action of the brain on the body is so violent—the overtaxing 
of the nervous system is so apt to prostrate the heart and 
derange the digestion; that the incapacities caused in these 
cases, are probably often due more to constitutional dis- 
turbance than to the direct deduction which excessive action 
entails. Such instances harmonize with the hypothesis; but 
how far they yield it positive support we cannot say. 


§ 368. An objection must here be guarded against. It is 
likely to be urged that since the civilized races are, on the 
average, larger than many of the uncivilized races ; and since 
they are also somewhat more complex as well as more active ; 
they ought, in conformity with the alleged general law, to 
be less prolific. There is, however, no evidence to prove that 
they are so: on the whole, they seem rather the reverse. 

The reply is that were all other things equal, these 
superior varieties of men should have inferior rates of in- 
crease. But other things are not equal; and it is. to the 
inequality of other things that this apparent anomaly is 
attributable. Already we have seen how much more fertile 
domesticated animals are than their wild kindred; and the 
causes of this greater fertility are also the causes of the 
greater fertility, relative or absolute, which civilized men 
exhibit when compared with savages. 

There is the difference in amount of food. Australians, 
Fuegians, and sundry races that might be named as having 
low rates of multiplication, are obviously underfed. The 
sketches of natives contained in the volumes of Livingstone, 
Baker, and others, yield clear proofs of the extreme 
depletion common among the uncivilized. In 
quality as well as in quantity, their feeding is bad. Wild 
fruits, insects, larvae, vermin, &c.,. which we refuse with 
disgust, often enter largely into their dietary. Much of this 
inferior food they eat uncooked; and they have not our 


488 LAWS OF MULTIPLICATION. 


elaborate appliances for mechanically-preparing it, and 
rejecting its useless parts. So that they live on matters of 
less nutritive value, which cost more both to masticate and 
to digest. Further, to uncivilized men supplies of 
food come very irregularly: long periods of scarcity are 
divided by short periods of abundance. And though by 
gorging when opportunity occurs, something is done towards 
compensating for previous want, yet the effects of prolonged 
starvation cannot be neutralized by occasional enormous 
meals. Bearing in mind, too, that improvident as they are, 
savages often bestir themselves only under pressure of 
hunger, we may fairly consider them as habitually ill- 
nourished—may see that even the poorer classes of civilized 
men, making regular meals on food separated from in- 
nutritive matters, easy to masticate and digest, tolerably 
good in quality and adequate if not abundant in quantity, 
are much better nourished. 

Then, again, though a much greater consumption in mus- 
cular action appears to be undergone by civilized men than 
by savages; and though it is probably true that among our 
labouring people the daily repairs cost more; yet in many 
cases there does not exist so much difference as we are apt to 
suppose. The chase is very laborious; and great amounts of 
exertion are gone through by the lowest races in seeking 
and securing the odds and ends of wild food on which 
they largely depend. We naturally assume that because 
barbarians are averse to regular labour, their muscular 
action is less than our own. But this is not necessarily true. 
The monotonous toil is what they cannot tolerate; and they 
may be ready to go through as much or more exertion when 
it is joined with excitement. If we remember that the 
sportsman who gladly scrambles up and down rough hill- 
sides all day after grouse or deer, would think himself hardly 
used had he to spend as much effort and time in digging ; we 
shall see that a savage who is the reverse of industrious, 
may nevertheless be subject to a muscular waste not very 


MULTIPLICATION OF THE HUMAN RACE. 489 


different in amount from that undergone by the indus- 
trious. When it is added that a larger physiolo- 
gical expenditure is entailed on the uncivilized than on the 
civilized by the absence of good appliances for shelter and 
proteetion—that in some cases they have to make good a 
greater loss of heat, and in other cases suffer much wear from 
irritating swarms of insects—we shall see that the total cost 
of self-maintenance among them is probably in many cases 
little less, and in some cases more, than it is among ourselves. 

So that though, on the average, the civilized are probably 
larger than the savage; and though they are, in their 
nervous systems at least, somewhat more complex; and 
though, other things equal, they ought to be the less 
prolific; yet, other things are so unequal, as to make it 
quite conformable to the general law that they should be 
more prolific. In § 365 we observed how, among inferior 
animals, higher evolution sometimes makes self-preservation 
far easier, by opening the way to resources previously un- 
available: so involving an undiminished, or even an in- 
creased, rate of genesis. And similarly we may expect 
among races of men, that those whose slight further develop- 
ments have been followed by habits and arts that immensely 
facilitate life, will not exhibit a lower degree of fertility, and 
may even exhibit a higher. 


§ 369. One more objection has to be met—a kindred ob- 
jection to which there is a kindred reply. Cases may be 
named of men conspicuous for activity, bodily and mental, 
who were also noted, not for less generative power than usual, 
but for more. As their superiorities indicate higher degrees 
of evolution, if may be urged that such men should, accord- 
ing to the theory, have lower degrees of reproductive activity. 
The fact that here, along with increased powers of self-pre- 
servation, there go increased powers of race-propagation, 
seems irreconcilable with the general doctrine. Reconcilia 
tion is not difficult however, 


490 LAWS OF MULTIPLICATION. 


The cases are analogous to some before named, in which 
more abundant food simultaneously agerandizes the indi- 
vidual and adds to the production of new individuals—the 
difference between the cases being, that instead of a better 
external supply of materiais there is here a better internal 
utilization of materials. Creatures of the same species noto- 
riously differ in goodness of constitution. Here there is some 
visceral defect, showing itself in feebleness of all the func- 
tions; while here some peculiarity of organic balance, some 
high quality of tissue, some abundance or potency of the 
digestive juices, gives to the system a perpetual high tide of 
rich blood, that serves at once to enhance the vital activities 
and to raise the power of propagation. Such variations, 
however, are quite independent of changes in the proportion 
between Individuation and Genesis: this remains the same, 
while both are increased or decreased by the increase or 
decrease of the common stock of materials. 

An illustration will best clear up any perplexity. Let us 
say that the fuel burnt in the furnace of a locomotive steam- 
engine, answers to the food which a man consumes; let us 
say that the produced steam expended in working the engine, 
corresponds to that portion of absorbed nutriment which 
carries on the man’s functions and activities; and let us 
say that the steam blowing off at the safety-valve, 
answers to that portion of the absorbed nutriment which 
goes to the propagation of the race. Such being the condi- 
tions of the case, several kinds of variations are possible. 
All other circumstances remaining the same, there may be 
changes of proportion between the steam used for working 
the engine and the steam that escapes by the safety-valve. 
There may be a structural or organic change of proportion. 
By enlarging the safety-valve or weakening its spring, while 
the cylinders are reduced in size, there may be established a 
constitutionally-small power of locomotion and a constitu- 
tionally-large amount of escape-steam ; and inverse variations 
so produced, will answer to the inverse variations between 


MULTIPLICATION OF THE HUMAN RACE. 49] 


Individuation and Genesis which different types of organisms 
show us. Again, there may be a functional change of pro- 
portion. If the engine has to draw a considerable load, the 
abstraction of steam by the cylinders greatly reduces the 
discharge by the safety-valve; and if a high velocity is kept 
up, the discharge from the safety-valve entirely ceases. Con- 
versely, if the velocity is low, the escape-steam bears a large 
ratio to the steam consumed by the motor apparatus; and if 
the engine becomes stationary the whole of the steam escapes 
by the safety-valve. This inverse variation answers to that 
which we have traced between Expenditure and Genesis, as 
displayed in the contrasts between species of the same type 
but unlike activities, and in the contrasts between active and 
inactive individuals of the same species. But now beyond 
these inverse variations between the quantities of consumed 
steam and escape-steam, that are structurally and functionally 
caused, there are coincident variations, producible in both by 
changes in the quantity of steam supplied—changes that 
may be caused in several ways. In the first place, the fuel 
thrown into the furnace may be increased or made better. 
Other things equal, there will result a more active locomo- 
tion as well as a greater escape; and this will answer to that 
simultaneous addition to its individual vigour and its repro- 
ductive activity, caused in an animal by a larger quantity, or 
a superior quality, of food. In the second place, the steam 
generated may be economized. Loss by radiation from the 
boiler may be lessened by a covering of non-conducting sub- 
stances; and part of the steam thus prevented from con- 
densing, will go to increase the working power of the engine, 
while part wilt be added to the quantity blowing off. This 
variation correspends to that simultaneous addition to bodily 
vigour and propagative power, which results in animals that 
have to expend less in keeping up their temperatures. In 
the third place, by improvement of the steam-generating 
apparatus, more steam may be obtained from a given weight 
of fuel. A better-formed evapcrating surface, or boiler plates 


492 LAWS OF MULTIPLICATION 


which conduct more rapidly, or an increased number of tubes, 
may cause a larger absorption of heat from the burning mass 
or the hot gases it gives off; and the extra steam generated 
by this extra heat, will, as before, augment both the motive 
force and the emission through the safety-valve. And this 
last case of coincident variation, is parallel to the case with 
which we are here concerned—the augmentation of indi- 
vidual expenditure and of reproductive energy, that may be 
caused by a superiority of some organ on which the utilizing 
or economizing of materials depends. 

Manifestly, therefore, an increased expenditure for Genesis, 
or an increased expenditure for Individuation, may arise in 
one of two quite different ways—either by diminution of the 
antagonistic expenditure, or by addition to the store which 
supplies both expenditures; and confusion results from not 
distinguishing between these. Given the ratio 4 to 20, as 
expressive of the relative costs of Genesis and Individuation, 
and the expenditure for Genesis may be raised to 5 while the 
expenditure for Individuation is raised to 25, without any 
alteration of type; merely by favourable circumstances or 
superiority of constitution. On the other hand, circumstances 
remaining the same, the expenditure for Genesis may be 
raised from 4 to 5, by lowering the expenditure for Indi- 
viduation from 20 to 19: which change of ratio may be 
either functional and temporary, or structural and per- 
manent. And only when it is the last does it illustrate that 
inverse variation between degree of evolution and degree of 
procreative dissolution, which we have everywhere seen. 


§ 370. There is no reason to suppose, then, that the laws 
of multiplication which hold of other beings, do not hold of 
the human being. On the contrary, there are special facts 
which unite with general implications, to show that these 
laws do hold of the human being. The absence of direct 
evidence in some cases where it might be looked for, we find 
fully explained when all the factors are taken into account. 


MULTIPLICATION OF THE HUMAN RACE. 493 


And certain seemingly-adverse facts, prove, on examination, 
to be facts belonging to a different category from that in 
which they are placed, and harmonize with the rest when 
rightly interpreted. 

The conformity of human fertility to the laws of multipli- 
cation in general, being granted, it remains to inquire what 
effects must be caused by permanent changes in men’s natures 
and circumstances. Thus far we have observed how, by their 
extremely-high evolution and extremely-low fertility, man- 
kind display the inverse variation between Individuation and 
Genesis, in one of its extremes. And we have also observed 
how mankind, like other kinds, are functionally changed in 
their rates of multiplication by changes of conditions. But 
we have not observed how alteration of structure in Man 
entails alteration of fertility. The influence of this factor is 
so entangled with the influences of other factors which are 
for the present more important, that we cannot recognize it. 
Here, if we proceed at all, we must proceed deductively. 


CHAPTER XIII. 
HUMAN POPULATION IN THE FUTURE. 


§ 371. Any further evolution in the most-highly evolved 
of terrestrial beings, Man, must be of the same nature as 
evolution in general. Structurally considered, it may consist 
in greater integration, or greater differentiation, or both— 
augmented bulk, or increased heterogeneity and definiteness, 
or a combination of the two. Functionally considered, it 
may consist in a larger sum of actions, or more multiplied 
varieties of actions, or both—a larger amount of sensible and 
insensible motion generated, or motions more numerous in 
kind and more intricate and exact in co-ordination, or 
motions that are greater alike in quantity, complexity, and 
precision. 

Expressing the change in terms of that more special 
evolution displayed by organisms ; we may say that it must 
be one which further adapts the moving equilibrium of 
organic actions. As it was pointed out in First Principles, 
§ 138, “the maintenance of such a moving equilibrium, re- 
quires the habitual genesis of internal forces corresponding 
in number, directions, and amounts to the external incident 
forces—as many inner functions, single or combined, as there 
are single or combined outer actions to be met.” And it 
was also pointed out that “‘ the structural complexity accom- 
panying functional equilibration, is definable as one in which 
there are as many specialized parts as are capable, separately 


HUMAN POPULATION IN THE FUTURE. 495 


and jointly, of counteracting the separate and joint forces 
amid which the organism exists.” Clearly, then, since all 
incompletenesses in Man as now constituted, are failures to 
meet certain of the outer actions, mostly involved, remote, 
irregular, to which he is exposed; every advance implies 
additional co-ordinations of actions and accompanying com- 
plexities of organization. 

Or once more, to specialize still further this conception of 
future progress, we may consider it as an advance towards 
completion of that continuous adjustment of internal to ex- 
ternal relations, which constitutes Life. In Part I. of this 
work, where it was shown that the correspondence between 
inner and outer actions called Life, is a particular kind of 
what, in terms of Evolution, we called a moving equilibrium ; 
it was shown that the degree of life varies as the degree of 
correspondence. Greater evolution or higher life, implies, 
then, such modifications of human nature as shall make more 
exact the existing correspondences, or shall establish addi- 
tional correspondences, or both. Connexions of phenomena 
of a rare, distant, unobtrusive, or intricate kind, which we 
either suffer from or do not take advantage of, have to be 
responded to by new connexions of ideas, and acts properly 
combined and proportioned: there must be increase of know- 
ledge, or skill, or power, or of all these. And to effect this 
more extensive, more varied, and more accurate, co-ordina- 
tion of actions, there must be organization of still greater 
heterogeneity and definiteness. 


§ 372. Let us before proceeding, consider in what par- 
ticular ways this further evolution, this higher life, this 
greater co-ordination of actions, may be expected to show 
itself. 

Will it be in strength? Probably not to any considerable 
degree. Mechanical appliances are fast supplanting brute 
force, and doubtless will continue doing this. Though at 
present civilized nations largely depend for self-preservation 


496 LAWS OF MULTIPLICATION. 


on vigour of limb, and are likely to do so while wars con- 
tinue; yet that progressive adaptation to the social state which 
must at last bring wars to an end, will leave the amount of 
muscular power to adjust itself to the requirements of a 
peaceful regime. Though, taking all things into account, the 
muscular power then required may not be less than now, 
there seems no reason why more should be required. 

Will it be in swiftness or agility? Probably not. In the 
savages these are important elements of the ability to main- 
tain life; but in the civilized man they aid self-preservation 
in quite a minor degree, and there seems no circumstance 
likely to necessitate an increase of them. By games and 
gymnastic competitions, such attributes may indeed be arti- 
ficially increased; but no artificial increase which does not 
bring a proportionate advantage can be permanent; since, 
other things equal, individuals and societies that devote the 
same amounts of energy in ways that subserve life more 
effectually, must by and by predominate. 

Will it be in mechanical skill, that is, in the better-co- 
ordination of complex movements? Most likely in some 
degree. Awkwardness is continually entailing injuries and 
deaths. Moreover, the complicated tools which civilization 
brings into use, are constantly requiring greater delicacy of 
manipulation. All the arts, industrial and esthetic, as they 
develop, imply a corresponding development of perceptive and 
executive faculties in men—the two necessarily act and react. 

Will it be in intelligence? Largely, no doubt. There is 
ample room for advance in this direction, and ample demand 
for it. Our lives are universally shortened by our ignorance. 
In attaining complete knowledge of our own natures and of 
the natures of surrounding things—in ascertaining the con- 
ditions of existence to which we must conform, and in dis- 
covering means of conforming to them under all variations 
of seasons and circumstances—we have abundant scope for 
intellectual progress 

Will it be in morality, that is, in greater power of self- 


HUMAN POPULATION IN THE FUTURE, 497 


regulation? Largely also: perhaps most largely. Right 
conduct is usually come short of more from defect of 
will than defect of knowledge. To the due co-ordination 
of those complex actions which constitute human life in 
its civilized form, there goes not only the pre-requisite 
—recognition of the proper course; but the further pre- 
requisite—a due impulse to pursue that course. And on 
calling to mind our daily failures to fulfil often-repeated 
resolutions, we shall perceive that lack of the needful desire, 
rather than lack of the needful insight, is the chief cause of 
faulty action. A further endowment of those feelings which 
civilization is developing in us—sentiments responding to 
the requirements of the social state—emotive faculties that 
find their gratifications in the duties devolving on us—must 
be acquired before the crimes, excesses, diseases, improvi- 
dences, dishonesties, and cruelties, that now so greatly 
diminish the duration of life, can cease. 

Thus, looking at the several possibilities, and asking 
what direction this further evolution, this more complete 
moving equilibrium, this better adjustment of inner to 
outer relations, this more perfect co-ordination of actions, 
is likely to take; we conclude that it must take mainly the 
direction of a higher intellectual and emotional develop-' 
ment. 


§ 373. This conclusion we shall find equally forced on us 
if we inquire for the causes which are to bring about such 
results. No more in the case of Man than in the case of any 
other being, can we presume that evolution either has taken 
place, or will hereafter take place, spontaneously. In the 
past, at present, and in the future, all modifications, func- 
tional and organic, have been, are, and must be immediately 
or remotely consequent on surrounding conditions, What, 
then, are those changes in the environment to which, by direct 
or indirect equilibration, the human organism has been 


adjusting itself, is adjusting itself now, and will continue to 
VOL. I. 32 


498 LAWS OF MULTIPLICATION. 


adjust itself? And how do they necessitate a higher evolu- . 
tion of the organism ? 

Civilization, everywhere having for its antecedent the in- 
crease of population, and everywhere having for one of its 
consequences a decrease of certain race-destroying forces, has 
for a further consequence an increase of certain other race- 
destroying forces. Danger of death from predatory animals 
lessens as men grow more numerous. Though, as they spread 
over the Earth and divide into tribes, men become wild 
beasts to one another, yet the danger of death from this 
cause also diminishes as tribes coalesce into nations. But the 
danger of death which does not diminish, is that produced by 
augmentation of numbers itself—the danger from deficiency 
of food. Supposing human nature to remain unchanged, the 
mortality hence resulting would, on the average, rise as 
human beings multiplied. If mortality, under such condi- 
tions, does not rise, it must be because the supply of food © 
also augments; and this implies some change in human 
habits wrought by the stress of human needs. Here, then, is 
the permanent cause of modification to which civilized men 
are exposed. Though the intensity of its action is ever being 
mitigated in one direction, by greater production of food ; it 
is, in the other direction, ever being added to by the greater 
production of individuals. Manifestly, the wants of their 
redundant numbers constitute the only stimulus mankind 
have to obtain more necessaries of life: were not the demand 
beyond the supply, there would be no motive to increase the 
supply. And manifestly, this excess of demand over supply 
is perennial: this pressure of population, of which it is the 
index, cannot be eluded. Though by the emigration that 
takes place when the pressure arrives at a certain intensity, 
temporary relief is from time to time obtained; yet as, by 
this process, all habitable countries must become peopled, it 
follows that in the end, the pressure, whatever it may then 
be, must be borne in full. 

This constant increase of people beyond the means of sub- 


HUMAN POPULATION IN THE FUTURE. 499 


sistence, causes, then, a never-ceasing requirement for skill, 
intelligence, and self-control—involves, therefore, a constant 
exercise of these and gradual growth of them. Every 
industrial improvement is at once the product of a higher form 
of humanity, and demands that higher form of humanity to 
carry it into practice. The application of science to the arts, 
is the bringing to bear greater intelligence for satisfying our 
wants; and implies continued progress of that intelligence. 
To get more produce from the acre, the farmer must study 
chemistry, must adopt new mechanical appliances, and must, 
by the multiplication of processes, cultivate both his own 
powers and the powers of his labourers. To meet the 
requirements of the market, the manufacturer is_per- 
petually improving his old machines, and inventing new 
ones ; and by the premium of high wages incites artizans to 
acquire greater skill. The daily-widening ramifications of 
commerce entail on the merchant a need for more know- 
ledge and more complex calculations; while the lessening 
profits of the ship-owner force him to build more scientifi- 
cally, to get captains of higher intelligence, and better crews. 
In all cases, pressure of population is the original cause. 
Were it not for the competition this entails, more thought 
and energy would not daily be spent on the business of life ; 
and growth of mental power would not take place. 
Difficulty in getting a living is alike the incentive to a 
higher education of children, and to a more intense and 
long-continued application in adults. In the mother it in- 
duces foresight, economy, and skilful house-keeping ; in the 
father, laborious days and constant self-denial. Nothing but 
necessity could make men submit to this discipline; and 
nothing but this discipline could produce a continued pro- 
gression. | 

In this case, as in many others, Nature secures each step 
in advance by a succession of trials; which are perpetually 
repeated, and cannot fail to be repeated, until success is 


achieved. All mankind in turn subject themselves more or 
Sah 


500 LAWS OF MULTIPLICATION. 


less to the discipline described ; they either may or may not 
advance under it; but, in the nature of things, only those 
who do advance under it eventually survive. For, neces- 
sarily, families and races whom this increasing difficulty of 
getting a living which excess of fertility entails, does rot 
stimulate to improvements in production—that is, to greater 
mental activity—are on the high road to extinction; and 
must ultimately be supplanted by those whom the pressure 
does so stimulate. This truth we have recently seen exem- 
plified in Ireland. And here, indeed, without further 
illustration, it will be seen that premature death, under all 
its forms and from all its causes, cannot fail to work in the 
same direction. For as those prematurely carried-off must, 
in the average of cases, be those in whom the power of self- 
preservation is the least, it unavoidably follows that those 
left behind to continue the race, must be those in whom 
the power of self-preservation is the greatest—must be the 
select of their generation. {So that, whether the dangers to 
existence be of the kind produced by excess of fertility, or of 
any other kind, it is clear that by the ceaseless exercise of 
the faculties needed to contend with them, and by the death 
of all men who fail to contend with them successfully, there 
is ensured a constant progress towards a higher degree of 
skill, intelligence, and self-regulation—a better co-ordina- 
tion of actions—a more complete life.* 


* A good deal of this chapter retains its original form; and the above 
paragraph is reprinted verbatim from the Westminster Review for April, 1852, 
in which the views developed in the foregoing hundred pages were first 
sketched out. This paragraph shows how near one may be toa great generaliza- 
tion without seeing it. Though the process of natural selection is recognized ; 
and though to it is ascribed a share in the evolution of a higher type; yet the 
conception must not be confounded with that which Mr. Darwin has worked 
out with such wonderful skill, and supported by such vast stores of knowledge. 
In the first place, natural selection is here described only as furthering direct 
adaptation—only as aiding progress by the preservation of individuals in 
whom functionally-produced modifications have gone on most favourably. In 
the second place, there is no trace of the idea that natural selection may, by 
co-operation with the cause assigned, or with other causes, produce divergences 


HUMAN POPULATION IN THE FUTURE. 501 


§ 374. The proposition at which we have thus arrived, is, 
then, that excess of fertility, through the changes it is ever 
working in Man’s environment, is itself the cause of Man’s 
further evolution; and the obvious corollary here to be 
drawn, is, that Man’s further evolution so brought about, 
itself necessitates a decline in his fertility. 

That future progress of civilization which the never- 
ceasing pressure of population must produce, will be ac- 
companied by an enhanced cost of Individuation, both in 
structure and function; and more especially in nervous 
structure and function. The peaceful struggle for existence 
in societies ever growing more crowded and more complicated, 
must have for its concomitant an increase of the great nervous 
centres in mass, in complexity, in activity. The larger body 
of emotion needed as a fountain of energy for men who have 
to hold their ‘places and rear their families under the inten- 
sifying competition of social life, is, other things equal, the 
correlative of larger brain. Those higher feelings presupposed 
by the better self-regulation which, in a better society, can 
alone enable the individual to leave a persistent posterity, are, 
other things equal, the correlatives of a more complex brain; 
as are also those more numerous, more varied, more general, 
and more abstract ideas, which must also become increasingly 


of structure; and of course, in the absence of this idea, there is no ime 
plication, even, that natural selection has anything to do with the origin uf 
species. And in the third place, the all-important factor of variation— 
‘* spontaneous,” or incidental as we may otherwise call it—is wholly ignored. 
Though use and disuse are, I think, much more potent causes of organic 
modification than Mr. Darwin supposes—though, while pursuing the inquiry 
in detail, I have been led to believe that direct equilibration has played a 
more active part even than I had myself at one time thought ; yet I hold 
Mr. Darwin to have shown beyond question, that a great part of the facts— 
perhaps the greater part—are explicable only as resulting from the survival of 
individuals which have deviated in some indirectly-caused way from the 
ancestral type. Thus, the above paragraph contains merely a passing recogni- 
tion of the selective process; and indicates no suspicion of the enormous 
range of its effects, or of the conditions under which a large part of its effects 


are produced. 


902 LAWS OF MULTIPLICATION. 


requisite for successful life as society advances. And the 
genesis of this larger quantity of feeling and thought, in a 
brain thus augmented in size and developed in structure, is, 
other things equal, the correlative of a greater wear of nerv- 
ous tissue and greater consumption of materials to repair it. 
So that both in original cost of construction and in subse- 
quent cost of working, the nervous system must become a 
heavier tax on the organism. Already the brain of the civi- 
lized man is larger by nearly thirty per cent. than the brain 
of the savage. Already, too, it presents an increased hetero- 
geneity—especially in the distribution of its convolutions. 
And further changes like these which have taken place 
under the discipline of civilized life, we infer will continue 
to take place. But everywhere and always, evolu- 
tion is antagonistic to procreative dissolution. Whether it 
be in greater growth of the organs which subserve self-main- 
tenance, whether it be in their added complexity of structure, 
or whether it be in their higher activity, the abstraction of 
the required materials, implies a diminished reserve of ma- 
terials for race-maintenance. And we have seen reason to 
believe that this antagonism between Individuation and 
Genesis, becomes unusually marked where the nervous sys- 
tem is concerned, because of the costliness of nervous struc- 
ture and function. In § 3846 was pointed out the apparent 
connexion between high cerebral development and _pro- 
longed delay of sexval maturity; and in § § 366, 367, 
the evidence went to show that where exceptional fer- 
tility exists there is sluggishness of mind, and that where 
there has been during education excessive expenditure in 
mental action, there frequently follows a complete or partial 
infertility. Hence the particular kind of further evolution 
which Man is hereafter to undergo, is one which, more than 
any other, may be expected to cause a decline in his power of 
reproduction. 

The higher nervous development and greater expenditure 
in nervous action, here described as indirectly brought about 


HUMAN POPULATION IN THE FUTURE. 503 


by increase of numbers, and as thereafter becoming a check 
on the increase of numbers, must not be taken to imply 
an intenser strain—a mentally-laborious life. The greater 
emotional and intellectual power and activity above con- 
templated, must be understood as becoming, by small incre- 
ments, organic, spontaneous and pleasurable. As, even when 
relieved from the pressure of necessity, large-brained Euro- 
peans voluntarily enter on enterprises and activities which 
the savage could not keep up even to satisfy urgent wants ; 
so, their still larger-brained descendants will, in a still higher 
degree, find their gratifications in careers entailing still 
greater mental expenditures. This enhanced demand for 
materials to establish and carry on the psychical functions, 
will be a constitutional demand. We must conceive the 
type gradually so modified, that the more-developed nervous 
system irresistibly draws off, for its normal and unforced 
activities, a larger proportion of the common stock of nutri- 
ment; and while so increasing the intensity, completeness, 
and length of the individual life, necessarily diminishing the 
reserve applicable to the setting up of new lives—no longer 
required to be so numerous. 

Though the working of this process will doubtless be 
interfered with and modified in the future, as it has been in 
the past, by the facilitation of living which civilization 
brings; yet nothing beyond temporary interruptions can so 
be caused.. However much the industrial arts may be im 
proved, there must be a limit to the improvement; while, 
with a rate of multiplication in excess of the rate of mortality, 
population must continually tread on the heels of produc- 
tion. So that though, during the earlier stages of civiliza- 
tion, an increased amount of food may accrue from a given 
amount of labour; there must come a time when this relation 
will be reversed, and when every additional increment of 
food will be obtained by a more than proportionate labour : 
the disproportion growing ever higher, and the diminution 
of the reproductive power becoming greater. 


504 LAWS OF MULTIPLICATION. 


§ 875. There now remains but to inquire towards what 
limit this progress tends. So long as the fertility of the 
race is more than sufficient to balance the diminution by 
deaths, population must continue to increase. So long as 
population continues to increase, there must be pressure on 
the means of subsistence. And so long as there is pressure 
on the means of subsistence, further mental development must 
go on, and further diminution of fertility must result Thus, 
the change can never cease until the rate of multiplication 1s 
just equal to the rate of mortality; that is, can never cease 
until, on the average, each pair has as many children as are 
requisite to produce another generation of child-bearing 
adults, equal in number to the last generation. At first 
sight, this would seem to imply that eventually each pair will 
rarely have more than two offspring ; but a little considera- 
tion shows that this is a lower degree of fertility than is 
likely ever to be reached. | 

Supposing the Sun’s light and heat, on which all terres- 
trial life depends, to continue abundant, for a period long 
enough to allow the entire evolution we are contemplating ; 
there are still certain slow astronomic and geologic changes 
which must prevent such complete adjustment of human nature 
to surrounding conditicas, as would permit the rate of mul- 
tiplication to fall so low. As before pointed out (§ 148) 
during an epoch of 21,000 years, each hemisphere goes 
through a cycle of temperate seasons and seasons extreme in 
their heat and cold — variations that are themselves alternately 
exaggerated and mitigated in the course of far longer cycles ; 
and we saw that these caused perpetual ebbings and 
flowings of species over different parts of the Earth’s surface. 
Further, by slow but inevitable geologic changes, especially 
those of elevation and subsidence, the climate and physical 
characters of every habitat are modified; while old habitats 
are destroyed and new are formed. This, too, we noted as 
a constant cause of migrations and of consequent alterations 
of environment. Now though the human race differs from 


HUMAN POPULATION IN THE FUTURE. 505 


other races in having a power of artificially counteracting 
external changes, yet there are limits to this power; and, 
even were there no limits, the changes could not fail to 
work their effects indirectly, if not directly. If, as is thought 
probable, these astronomic cycles entail recurrent glacial pe- 
riods in each hemisphere, then, parts of the Earth that are at 
one time thickly peopled, will at another time, be almost de- 
serted, and vice versd. The geologically-caused alterations of 
climate and surface, must produce further slow re-distributions 
of population; and other currents of people, to and from different 
regions, will be necessitated by the rise of successive centres 
of higher civilization. Consequently, mankind cannot but 
continue to undergo changes of environment, physical and 
moral, analogous to those which they have thus far been 
undergoing. Such changes may eventually become slower 
and less marked; but they can never cease. And if they can 
never cease, there can never arise a perfect adaptation of 
human nature to its conditions of existence. To establish 
that complete correspondence between inner and outer actions 
which constitutes the highest life and greatest power of self- 
preservation, there must be a prolonged converse between the 
organism and circumstances that remain the same. If the 
external relations are being altered while the internal rela- 
tions are being adjusted to them, the adjustment can never 
become exact. And in the absence of exact adjustment, 
there cannot exist that theoretically-highest power of self- 
preservation with which there would co-exist the theoretically- 
lowest power of race-production. 

Hence though the number of premature deaths may ul- 
timately become very small, it can never become so small 
as to allow the average number of offspring from each pair 
to fall so low as two. Some average number between two 
and three may be inferred as the limit—a number, however, 
that is not likely to be quite constant, but may be ex- 
pected at one time to increase somewhat and afterwards 
to decrease somewhat, according as variations in physical 


506 LAWS OF MULTIPLICATION. 


and social conditions lower or raise the cost of self- 
preservation. 

Be this as it may, however, it is manifest that in the pis 
pressure of population and its accompanying evils will dis- 
appear; and will leave a state of things requiring from each 
individual no more than a normal and pleasurable activity. 
Cessation in the decrease of fertility implies cessation in 
the development of the nervous system; and this implies a 
nervous system that has become equal to all that is demanded 
of it—has not to do more than is natural to it. But that 
exercise of faculties which does not exceed what is natural, 
constitutes gratification. In the end, therefore, the ob- 
tainment of subsistence and discharge of all the parental 
and social duties, will require just that kind and that amount 
of action needful to health and happiness. 

The necessary antagonism of Individuation and Genesis, 
not only, then, fulfils with precision the @ priori law of 
maintenance of race, from the Monad up to Man, but ensures 
final attainment of the highest form of this maintenance— 
a form in which the amount of life shall be the greatest 
possible, and the births and deaths the fewest possible. This 
antagonism could not fail to work out the results we see it 
working out. The excess of fertility has itself rendered the 
process of civilization inevitable ; and the process of civiliza- 
tion must inevitably diminish fertility, and at last destroy its 
excess. From the beginning, pressure of population has 
been the proximate cause of progress. It produced the 
original diffusion of the race. It compelled men to abandon 
predatory habits and take to agriculture. It led to the 
clearing of the Earth’s surface. It foreed men into the 
social state; made social organization inevitable; and has 
developed the social sentiments. It has stimulated to pro- 
gressive lmprovements in production, and to increased skill 
and intelligence. It is daily thrusting us into closer contact 
and more mutually-dependent relationships. And after having 
caused, as it ultimately must, the due peopling of the globe, 


HUMAN POPULATION IN THE FUTURE. 507 


and the raising of all its habitable parts into the highest 
state of culture—after having brought all processes for the 
satisfaction of human wants to perfection—after having, at 
the same time, developed the intellect into complete com- 
petency for its work, and the feelings into complete fitness 
for social life—after having done all this, the pressure of 
population, as it gradually finishes its work, must gradually 
bring itself to an end. 


§ 377. In closing the argument let us not overlook the 
self-sufficingness of those universal processes by which the 
results reached thus far have been wrought out, and which 
may be expected to work out these future results. 

Evolution under all its aspects, general and special, is an 
advance towards equilibrium. We have seen that the theo- 
retical limit towards which the integration and differentia- 
tion of every aggregate advances, is a state of balance be- 
tween all the forces to which its parts are subject, and 
the forces which its parts oppose to them (First Prin. § 180). 
And we have seen that organic evolution is a progress 
towards a moving equilibrium completely adjusted to en- 
vironing actions. 

It has been also pointed out that, in civilized Man, there is 
going on a new class of equilibrations—those between his ac- 
tions and the actions of the societies he forms (First Prin. 
§ 185). Social restraints and requirements are ever altering 
his activities and by consequence his nature ; and as fast as his 
nature is altered, social restraints and requirements undergo 
more or less re-adjustment. Here the organism and the con- 
ditions are both modifiable; and by successive conciliations 
of the two, there is effected a progress towards equilibrium. 

More recently we have seen that in every species, there 
establishes itself an equilibrium of an involved kind between 
the total race-destroying forces and the total race- preserving 
forces—an equilibrium which implies that where the ability 
to maintain individual life is small, the ability to propagate 


508 . LAWS OF MULTIPLICATION. 


must be great, and vice versd. Whence it follows that the 
evolution of a race more in equilibrium with the environment, 
is also the evolution of a race in which there is a correlative 
approach towards equilibrium between the number of new 
individuals produced and the number which survive and 
propagate. 

The final result to be observed, is, that in Man, all these 
equilibrations between constitution and. conditions, between 
the structure of society and the nature of its members, be- 
tween fertility and mortality, advance simultaneously towards 
a common climax. In approaching an equilibrium between his 
nature and the ever-varying circumstances of his inorganic 
environment, and in approaching an equilibrium between his 
nature and all the requirements of the social state, Man is at 
the same time approaching that lowest limit of fertility at 
which the equilibrium of population is maintained by the 
addition of as mapy infants as there are subtractions by death 
in old age. Changes numerical, social, organic, must, by their 
mutual influences, work unceasingly towards a state of har- 
mony—a state in which each of the factors is just equal to its 
work. And this highest conceivable result must be wrought 
out by that same universal process which the simplest inor- 
ganic action illustrates. 


THR END. 


APPENDICES, 


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APPENDIX A. 


SUBSTITUTION OF AXIAL. FOR FOLIAR ORGANS IN PLANTS. 


I aprenD here the evidences referred to in §190. The most 
numerous and striking I have met with among the Umbellifere. 
Monstrosities having the alleged implication, are frequent in the 
common Cow-Parsnep—so frequent that they must be familiar to 
botanists ; and wild Angelica supplies many over-developments of 
like meaning. Omitting numerous cases of more or less significance, 
I will limit myself to two. 

One of them is that of a terminal umbel, in which nine of the outer 
umbellules are variously transformed—here a single flower being made 
monstrous by the development of some of its members into buds; 
there several such malformed flowers being associated with rays that 
bear imperfect umbeliules ; and elsewhere, flowers being replaced by 


7 


umbellules: some of which are perfect, and others imperfect only in 
the shortness of the flower-stalks. The annexed Fig. 69, represent- 
ing in a somewhat conventionalized way, a part of the dried speci- 


512 


men, will give an idea of this Angelica, At a is shown a single 
flower partially changed; in the umbellule marked 0, one of the 
rays bears a secondary umbellule; and there may be seen at c and 
d, several such over-developments. 

But the most conclusive instance is that of a Cow-Parsnep, in which 
a single terminal umbel, besides the transformations already men- 
tioned, exhibits higher-degrees of such transformations.* The com- 
ponents of this complex growth are ;—three central umbellules, ab- 
normal only in minor points ; one umbellule, external to these, which 
is partially changed into an umbel; one rather more out of the 
centre, which is so far metamorphosed as to be more an umbel than 
“an umbellule: nine peripheral clusters formed by the development 
of umbellules into umbels, some of which are partially compounded 
still further. Hxamined in detail, these structures present the fol- 
lowing facts :—1. The innermost umbellule is normal, save in having 
a peripheral flower of which one member (apparently a petal) is 
transformed into a flower-bud. 2. The next umbellule, not quite so 
central, has one of its peripheral flowers made monstrous by the 
growth of a bud from the base of the calyx. 38. The third of 
the central umbellules has two abnormal outer flowers. One of 
them carries a flower-bud on its edge, in place of a foliar member. 
The other is half flower and half umbellule: being composed 
of three petals, three stamens, and five flower-buds growing 
where the other petals and stamens should grow. 4. Outside 
of these umbellules comes one of the mixed clusters. Its five 
central flowers are normal. Surrounding these are several 
flowers transformed in different degrees: one having a stamen par- 
tially changed into a flower bud. And then, at the periphery of 
this mixed cluster, come three complete umbellules and an incom- 
plete one in which some petals and stamens of the original flower 
remain. 5. A mixed cluster, in which the umbel-structure pre- 
dominates, stands next. Its three central flowers are normal. 
Surrounding them are five flowers over-developed in various ways, 
like those already described. And on its periphery are seven 
complete umbellules in place of flowers; besides an incomplete 
umbellule that contains traces of the original flower, one of them 
being a petal imperfectly twisted up into a bud. 6. Of the nine 
external clusters, in which the development of simple into compound 
umbels is most decided, nearly all present anomalies. Three of them 
have each a central flower untransformed ; and in others, the central 


* For the information of those who may wish to examine metamorphoses 
of these kinds, I may here state that I have found nearly all the examples 
described, in the neighbourhood of the sea—the last-named, on the shore of 
Locheil, near Fort William. Whether it is that I have sought more dili- 
gently for cases when in such localities, or whether it is that the sea-air 
favours that excessive nutrition whence these transformations result, I am 
unable to say. 


513 


umbellule is composed of two, three, or four flowers. 7. But the 
most remarkable fact is, that in sundry of these peripheral clusters, 
resulting from the metamorphosis of simple umbels into compound 
umbels, the like metamorphosis is carried a stage higher. Some of 
the component rays, are themselves the bearers of compound umbels 


V9 dL Oh Ss : 8 
MAY B22 pf 
AVZaNd | Waa | 
mK 


Fa \ 8 
23 ae 
VAY 2 0 > SY\ , 
<a 
a) re 


instead of simple umbels. In Fig. 70, a portion of the dried speci- 
men is represented. ‘Two of the central umbellules are marked a 
and J; those marked c and d are mixed clusters; at ¢ and / are 
compound umbels replacing simple ones; and g shows one of the 
rays on which the over-development goes still further. 

Does not this evidence, enforced as it is by much more of like 
kind, go far to prove that foliar organs may be developed into axial 
organs? Even were not the transitional forms traceable, there would 
still, | think, be no other legitimate interpretation of the facts last 
detailed. The only way of eluding the conclusion here drawn, is by 
assuming that where a cluster of flowers replaces a single flower, it 
is because the axillary buds which hypothetically belong to the 
several foliar organs of the flower, become developed into axes; and 
assuming this, is basing an hypothesis on another hypothesis that is 
directly at variance with facts. The foliar organs of flowers do not 
bear buds in their axils; and it would never have been supposed 
that such buds are typically present, had it not been for that 
mistaken conception of “type” which has led to many other errors 
in Biology. Goethe writes: ‘Now as we cannot realize the idea 
of a leaf apart from the node out of .which it springs, or of a node 
without a bud, we may venture to infer,” &c. See here an example 
of a method of philosophizing not uncommon among the Germans. 

VOL, IL 33 


514 


The method is this—Survey a portion of the facts, and draw from 
them a general conception; project this general conception back 
into the objective world, as a mould in which Nature casts her 
products; expect to find it everywhere fulfilled; and allege poten- 
tial fulfilment where no actual fulfilment is visible. 

If instead of imposing our ideal forms on Nature, we are con- 
tent to generalize the facts as Nature presents them, we shall find 
no warrant for the morphological doctrine above enunciated. The 
only conception of type justified by the logic of science, is—that 
correlation of parts which remains constant under all modifications 
of the structure to be defined. To ascertain this, we must compare 
all these modifications, and note what traits are common to them. 
On doing so with the successive segments of a phenogamic axis, 
we are brought to a conclusion widely different from that of Goethe. 
Axillary buds are almost universally absent from the cotyledons; 
they are habitually present in the axils of fully-developed leaves 
higher up the axis ; they are often absent from leaves that are close 
to the flower; they are nearly always absent from the bracts ; absent 
from the sepais; absent from the petals; absent from the stamens ; 
absent from the carpels. Thus, out of eight leading forms which 
folia assume, one has the axillary bud and seven are without it. 
With these facts before us, it seems to me not difficult to “ realize 
the idea” “of a node without a bud.” If we are not possessed 
by a foregone conclusion, the evidence will lead us to infer, that 
each node bears a foliar appendage and may bear an axillary bud. _ 

Even, however, were it granted that the typical segment of a 
Phenogam includes an axillary bud, which must be regarded as 
always potentially present, no legitimate counter-interpretation of 
the monstrosities above described could thence be drawn. If when 
an umbellule is developed in place of a flower, the explanation is, 
that its component rays are axillary to the foliar organs of the 
flower superseded ; we may fairly require that these foliar organs to 
which they are axillary, shall be shown. But there are none. In 
the last specimen figured, the inner rays of each such umbellule are 
without them; most of the outer rays are also without them; and 
in one cluster, only a single ray has a bract at its point of origin. 
There is a rejoinder ready, however: the foliar organs are said to 
be suppressed. Though Goethe could not “ realize the idea” “ of 
a node without a bud,” those who accept his typical form appear to 
find no difficulty in realizing the idea of an axillary bud without 
anything to which it is axillary. But letting this pass, suppose we 
ask what is the warrant for this assumed suppression. Axillary 
buds normally occur where the nutrition is high enough to produce 
fully-developed leaves; and when axillary buds are demonstrably 
present in flowers, they accompany foliar organs that are more leaf- 
like than usual—always greener if not always larger. That is to 


515 


say, the normal and the abnormal axillary buds, are alike the con- 
comitants of foliar organs coloured by that chlorophyll which 
habitually favours foliar development. How, then, can it be sup- 
posed that when, out of a flower there is developed a cluster of 
_ flower-bearing rays, the implied excess of nutrition causes the foliar 
organs to abort? It is true that very generally in a branched in- 
florescence, the bracts of the several flower-branches are very small 
(their smallness being probably due to that defective supply of 
certain chlorophyll-forming matters, which is the proximate cause 
of flowering) ; and it is true that, under these conditions, a flower- 
ing axis of considerable size, for the development of which chloro- 
phyll is less needful, grows from the axil of a dwarfed leaf. But 
the inference that the foliar organ may therefore be entirely sup- 
pressed, seems to me irreconcilable with the fact, that the foliar 
organ is always developed to some extent before the axillary bud 
appears. Until it has been shown that in some cases a lateral bud 
first appears, and a foliar organ afterwards grows out beneath it, to 
form its axil, the conception of an axillary bud of which the foliar 
organ is suppressed, will remain at variance with the established 
truths of development. 


The above originally formed a portion of $190. I have transferred 
it to the Appendix, partly because it contains too much detail to 
render it fit for the general argument, and partly because the inter- 
pretations being open to some question, it seemed undesirable to risk 
compromising that argument by including them. The criticisms 
passed upon these interpretations have not, however, sufficed to con- 
vince me of their incorrectness. Unfortunately, I have since had no 
opportunity of verifying the above statements by microscopic exami- 
nations, as I had intended. 

Though unable to enforce the inference drawn by further facts 
more minutely looked into, I may add some arguments based on 
facts that are well known. One of these is the fact that the so- 
called axillary bud is not universally axillary—is not universally 
seated in the angle made by the axis and an appended foliar organ. 
In certain plants the axillary bud is placed far above the node, 
half-way between it and the succeeding node. So that not only may 
a segment of a phsenogamic axis be without the axillary bud, but 
the axillary bud, when present, may be removed from that place in 
which, according to Goethe, it necessarily exists. Another fact not 
congruous with the current doctrine, is the common occurrence of 
“adventitious” buds—the buds that are put out from roots and from 
old stems or branches bare of leaves. The name under which they are 
thus classed, is meant to imply that they may be left out of conside 
ration. Those, however, who have not got a theory to save by 

33 * 


516 


putting anomalies out of sight, may be inclined to think that the 
occurrence of buds where they are avowedly unconnected with 
nodes, and are axillary to nothing, tells very much against the as- 
sumption that every bud implies a node and a corresponding foliar 
organ. And they may also see that the development of these ad- 
ventitious buds at places where there is excess of nutritive mate- 
rials, favours the view above set forth. For if a bud thus arises at 
a place where it is not morphologically accounted for, simply because 
there happens to be at that place an abundance of unorganized pro- 
toplasm ; then, clearly, it is likely that if the mass of protaplasm 
from which a ieaf would usually arise, is greatly increased in mass 
by excess of nutrition, it may develop into an axis instead of a leaf. 


ALBUS EIN Dit eer. 


A URITICISM ON PROF. OWEN’S THEORY OF 'THE 
VERTEBRATE SKELETON. 


[From the Bririsu & ForrtGN Mepico-CHiruRGICAL REVIEW For Ocr., 1858. ] 


I. On the Archetype and Homologies of the Vertebrate Skeleton. By 
Ricuarp Owen, /.2.S.—London, 1848. pp. 172. 


Il. Principes @Ostéologie Comparée, ou Recherches sur 0 Archétype 
et les Homologies du Squelette Vertébré. Par Ricnarp OwxEN.— 
Paris. 


Principles of Comparative Osteology ; or, Researches on the Archetype 
and the Homologies of the Vertebrate Skeleton. By Ricuarp 
OWEN. 


Til. On the Nature of Limbs. A Discourse delivered on Friday, 
February 9, at an Evening Meeting of the Royal Institution of 

- Great Britain. By Ricuarp Owen, F.R.S.—London, 1849. 
pp. 119. 


JupGine whether another proves his position is a widely different 
thing from proving your own. To establish a general law requires 
an extensive knowledge of the phenomena to be generalized; but to 
decide whether an alleged general law is established by the evidence 
assigned, requires merely an adequate reasoning faculty. Especially 
is such a decision easy where the premises do not warrant the con- 
clusion. It may be dangerous for one who has but little previous 
acquaintance with the facts, to say that a generalization is demon- 
strated ; seeing that the argument may be one-sided: there may be 
many facts unknown to him which disprove it. But it is not 
dangerous to give a negative verdict when the alleged demonstra- 


518 


tion is manifestly insufficient. If the data put before him do not 
bear out the inference, it is competent for every logical reader to 
say so. 

Krom this stand-point, then, we venture to criticize some of 
Professor Owen’s osteological theories. Far his knowledge of 
comparative osteology we have the highest respect. We believe 
that no living man has so wide and detailed an acquaintance with 
the bony structure of the Vertebrata. Indeed, there probably has 
never been any one whose information on the subject was so nearly 
exhaustive. Moreover, we confess that nearly all we know of this 
department of biology has been learnt from his lectures and writ- 
ings. We pretend to no independent investigations, but merely to 
such knowledge of the phenomena as he has furnished us with. 
Our position, then, is such that, had Professor Owen simply enun- 
ciated his generalizations, we should have accepted them on his 
authority. But he has brought forward evidence to prove them. 
By so doing he has tacitly appealed to the judgments of his readers 
and hearers—has practically said, “‘Here are the facts; do they 
not warrant these conclusions?” And all we propose to do, is to 
consider whether the conclusions are warranted by the facts brought 
forward. 

Let us first limit the scope of our criticisms. On that division 
of comparative osteology which deals with what Professor Owen 
distinguishes as “special homologies,” we do not propose to enter. 
That the wing of a bird is framed upon bones essentially parallel to 
those of a mammal’s fore-limb; that the cannon-bone of a horse’s 
leg answers to the middle metacarpal of the human hand; that 
various bones in the skull of a fish are homologous with bones in 
the skull of a man—these and countless similar facts, we take to he 
well established. It may be, indeed, that the doctrine of special 
homologies is at present carried too far. It may be that, just as 
the sweeping generalization at one time favoured, that the embryonic 
phases of the higher animals represent the adult forms of lower 
ones, has been found untrue in a literal sense, and is acceptable 
only in a qualified sense; so the sweeping generalization that the 
skeletons of all vertebrate animals consist of homologous parts, will 
have to undergo some modification. But that this generalization 
is substantially true, all comparative anatomists agree. 

The doctrine which we are here to consider, is quite a separate 
one—that of “general homologies.” The truth oy falsity of this 
may be decided on quite apart from that of the other. Whether 
certain bones in one vertebrate animal’s skeleton correspond with 
certain bones in another’s, or in every other’s, is one question; and 
whether the skeleton of every vertebrate animal is divisible into a 
series of segments, each of which is modelled after the same type, 
is another question. While the first is answered in the affirmative, 


519 


the last may be answered in the negative; and we propose to give 
reasons why it should be answered in the negative. 


In so far as his theory of the skeleton is concerned, Professor 
Owen is an avowed disciple of Plato. At the conclusion of his 
Archetype and Homologies of the Vertebrate Skeleton, he quotes ap- 
provingly the Platonic hypothesis of iéx:, “‘a sort of models, or 
moulds in which matter is cast, and which regularly produce the 
same number and diversity of species.” The vertebrate form in 
general (see diagram of the Archetypus), or else the form of each 
kind of vertebrate animal (see p. 172, where this seems implied), 
Professor Owen conceives to exist as an “idea”—an “ arche- 
typal exemplar on which it has pleased the Creator to frame 
certain of his living creatures.” Whether Professor Owen holds 
that the typical vertebra also exists as an “idea,” is not so 
certain. From the title given to his figure of the ‘ideal typical 
vertebra,” it would seem that he does; and at p. 40 of his 
Nature of Limbs, and indeed throughout his general argument, this 
supposition is implied. But on the last two pages of the Archetype 
and Homologies, it is distinctly alleged that “ the repetition of simi- 
lar segments in a vertebral column, and of similar elements in a 
vertebral segment, is analogous to the repetition of similar crystals 
as the result of polarizing force in the growth of an inorganic 
body ;” it is pointed out that, ‘“‘as we descend the scale of animal 
life, the forms of the repeated parts of the skeleton approach more 
and more to geometrical figures;” and it is inferred that “the 
Platonic idéa or specific organizing principle or force, would seem 
to be in antagonism with the general polarizing force, and to sub- 
due and mould it in subserviency to the exigencies of the resulting 
specific form.” If Professor Owen’s doctrine is to be understood 
as expressed in these closing paragraphs of his Archetype and Homo- 
logies—if he considers that ‘“ the idéz” ‘“ which produces the diver- 
sity of form belonging to living bodies of the same materials,” is 
met by the ‘ counter-operation” of ‘the polarizing force pervading 
all space,” which produces “the similarity of forms, the repetition 
of parts, the signs of unity of organization,” and which is “ subdued ” 
as we ascend ‘in the scale of being;” then we may pass on with 
the remark that the hypothesis is too cumbrous and involved to 
have much vraisemblance. If, on the other hand, Professor Owen 
holds, as every reader would suppose from the general tenor of his 
reasonings, that not only does there exist an archetypal or ideal 
vertebrate skeleton, but that there also exists an archetypal or 
ideal vertebra; then he carries the Platonic hypothesis much 
further than Plato does. Plato’s argument, that before any species 
of object was created it must have existed as an idea of the Creative 
Intelligence, and that hence all objects of such species must be 


520 


copies of this original idea, is tenable enough from the anthropo- 
morphic point of view. But while those who, with Plato, think fit 
to base their theory of creation upon the analogy of a carpenter 
designing and making a table, must yield assent to Plato’s inference, 
they are by no means committed to Professor Owen’s expansion of 
it. To say that before creating a vertebrate animal, God must 
have had the conception of one, does not involve saying that God 
gratuitously bound himself to make a vertebrate animal out of seg- 
ments all moulded after one pattern. As there is no conceivable 
‘advantage in this alleged adhesion to a fundamental pattern—as, 
for the fulfilment of the intended ends, it is not only needless, but 
oiten, as Professor Owen argues, less appropriate than some other 
construction would be (see Nature of Limbs, pp. 39, 40), to sup- 
pose the creative processes thus regulated, is not a little startling. 
Kven those whose conceptions are so anthropomorphic as to think 
they honour the Creator by calling him “ the Great Artificer,” will 
scarcely ascribe to him a proceeding which, in a human artificer, 
they would consider a not very worthy exercise of ingenuity. 

But whichever of these alternatives Professor Owen contends for 
—whether the typical vertebra is that more or less crystalline figure 
which osseous matter ever tends to assume in spite of “the id€a or 
organizing principle,” or whether the typical vertebra is itself an 
“ }iéx Or organizing principle”—there is alike implied the belief 
that the typical vertebra has an abstract existence apart from actual 
vertebre. It is a form which, in every endoskeleton, strives to 
embody itself in matter—a form which is potentially present in each 
vertebra; which is manifested in each vertebra with more or less 
clearness ; but which, in consequence of antagonizing forces, is no- 
where completely realized. Apart from the philosophy of this 
hypothesis, let us here examine the evidence which is thought to 
justify it. 


And first as to the essential constituents of the “ ideal typical 
vertebra.” Exclusive of “diverging appendages” which it ‘“ may 
also support,” ‘it consists in its typical completeness of the follow-. 
ing elements and parts”:—A centrum round which the rest are 
arranged in a somewhat radiate manner; above it two neuwrapophyses 
—converging as they ascend, and forming with the centrum a trian- 
guloid space containing the neural axis; a neural spine surmounting 
the two neurapophyses, and with them completing the neural arch ; 
below the centrum two hemapophyses and a hemal spine, forming a 
hemal arch similar to the neural arch above, and enclosing the 
hemal axis; two plewrapophyses radiating horizontally from the 
sides of the centrum; and two parapophyses diverging from the 
centrum below the pleurapophyses. . “ These,” says Professor 
Owen, “being usually developed from distinct and independent 


521 


999 


centres, I have termed ‘ autogenous elements. The remaining 
elements, which he classes as “ exogenous,” because they ‘ shoot 
out as coutinuations from some of the preceding elements,” are the 
diapophyses diverging from the upper part of the centrum as the 
parapophyses do below, and the zygapophyses which grow out of the 
distal ends of the neurapophyses and hemapophyses. 

If, now, these are the constituents of the vertebrate segment “ in 
its typical completeness ;” and if the vertebrate skeleton consists of 
a succession of such segments; we ought to have in these con- 
stituents, representatives of all the elements of the vertebrate 
skeleton—at any rate, all its essential elements. Are we then to 
conclude that the “‘ diverging appendages,” which Professor Owen 
regards as rudimental limbs, and from certain of which he considers 
actual limbs to be developed, are typically less important than some 
of the above-specified exogenous parts—say the zygapophyses ? 

That the meaning of this question may be understood, it will be 
needful briefly to state Professor Owen’s theory of Zhe Nature of 
Limbs; and such criticisms as we have to make on it must be in- 
cluded in the parenthesis. In the first place, he aims to show that 
the scapular and pelvic arches, giving insertion to the fore and hind 
limbs respectively, are displaced and modified hemal arches, 
originally belonging in the one case to the occipital vertebra, and in 
the other case to some trunk-vertebra not specified. In support of 
this assumption of displacement, carried in some cases to the extent 
of twenty-seven vertebrae, Professor Owen cites certain acknow- 
ledged displacements which occur in the human skeleton to the ex- 
tent of half a vertebra—a somewhat slender justification. But for 
proof that such a displacement Aas taken place in the scapular arch, 
he chiefly relies on the fact that in fishes, the pectoral fins, whick 
are the homologues of the fore-limbs, are directly articulated to 
certain bones at the back of the head, which he alleges are parts 
of the occipital vertebra. This appeal to the class of fishes is 
ayvowedly made on the principle that these lowest of the Vertebrata 
approach closest to archetypal regularity, and may therefore be 
expected to show the original relations of the bones more nearly. 
Simply noting the facts that Professor Owen does not give us any 
transitional forms between the alleged normal position of the 
scapular arch in fishes, and its extraordinary displacement in the 
higher Vertebrata ; and that he makes no reference to the embryonic 
phases of the higher Vertebrata, which might be expected to ex- 
hibit the progressive displacement; we go on to remark that, in 
the case of the pelvic arch, he abandons his principle of appealing 
to the lowest vertebrate forms for proof of the typical structure. 
In fishes, the rudimentary pelvis, widely removed from the spinal 
column, shows no signs of having belonged to any vertebra; and 
here Professor Owen instances the perennibranchiate Batrachia as 


522 


exhibiting the typical structure: remarking that ‘“ mammals, birds, 
and reptiles show the rule of connexion, and fishes the exception.” 
Thus in the case of the scapular arch, the evidence afforded by 
fishes is held of great weight, because of their archetypal regularity ; 
while in the case of the pelvic arch, their evidence is rejected as 
exceptional. But now, having, as he considers, shown that these 
bony frames to which the limbs are articulated are modified hemal 
arches, Professor Owen points out that the hemal arches habitually 
bear certain “ diverging appendages ;” and he aims to show that 
the “diverging appendages” of the scapular and pelvic arches re- 
spectively, are developed into the fore and hind limbs. ‘There are 
several indirect ways in which we may test the probability of this 
conclusion. If these diverging appendages are “ rudimental limbs ” 
—‘ future possible or potential arms, legs, wings, or feet,” we may 
fairly expect them always to bear to the hemal arches a relation 
such as the limbs do. But they by no means do this. ‘“ As the 
vertebre approach the tail, these appendages are often transferred 
gradually from the pleurapophysis to the parapophysis, or even to 
the centrum and neural arch.” (Arch. and Hom., p. 93.) Again, 
it might naturally be assumed that in the lowest vertebrate forms, 
where the limbs are but little developed, they would most clearly 
display their alliance with the appendages, or ‘‘ rudimental limbs,” 
by the similarity of their attachments. Instead of this, however, - 
Professor Owen’s drawings show that whereas the appendages are 
habitually attached to the pleurapophyses, the limbs, in their earliest 
and lowest phase, alike in fishes and in the Lepzdosiren, are articu- 
lated to the hemapophyses. Most anomalous of all, however, is 
the process of development. When we speak of one thing as being 
developed out of another, we imply that the parts next to the germ 
are the first to appear, and the most constant. In the evolution of 
a tree out of a seed, there come at the outset the stem and the 
radicle ; afterwards the branches and divergent roots; and still 
later the branchlets and rootlets ; the remotest. parts being the latest 
and most inconstant. If, then, a limb is developed out of a “ di- 
verging appendage ” of the hzemal arch, the earliest and most con- 
stant bones should be the humerus and femur; next in order of 
time and constancy should come the coupled bones based on these ; 
while the terminal groups of bones should be the last to make their 
appearance, and the most liable to be absent. Yet, as Professor 
Owen himself shows, the actual mode of development is the very re- 
verse of this. At p.16 of the Archetype and Homologies, he says :— 

‘¢ The earlier stages in the development of all locomotive extremities are 
permanently retained or represented in the paired fins of fishes. _ First the 
essential part of the member, the hand or foot, appears: then the fore-arm 
or leg, both much shortened, flattened, and expanded, as in all fins and all 


embryonic rudiments of limbs: finally come the humeral and femoral seg- 
ments ; but this stage I have not found attained in any fish,” 


523 


That is to say, alike in ascending through the Vertebrata gene- 
rally, and in tracing up the successive phases of a mammalian em- 
bryo, the last-developed and least constant division of the limb, is 
that basic one by which it articulates with the hemal arch. It 
seems to us that, so far from proving his hypothesis, Professor 
Owen’s own facts tend to show that limbs do not belong to the 
vertebrae at all; that they make their first appearance peripherally ; 
that their development is centripetal; and that they become fixed 
to such parts of the vertebrate axis as the requirements of the case 
determine. . 

But now, ending here this digressive exposition and criticism, 
and granting the position that limbs “are developments of costal 
appendages,” let us return to the question above put—Why are not 
these appendages included as elements of the “ ideal typical ver- 
tebra?” It cannot be because of their comparative inconstancy ; 
for judging from the illustrative figures, they seem to be as con- 
stant as the hemal spine, which is one of the so-called autogenous 
elements: in the diagram of the Archetypus, the appendage is re- 
presented as attached to every vertebrate segment of the head and 
trunk, which the hemal spine is not. It cannot be from their com- 
parative unimportance; seeing that as potential limbs they are 
essential parts of nearly all the Vertebrata—much more obviously 
so than the diapophyses are. If, as Professor Owen argues, “the 
divine mind which planned the archetype also foreknew all its 
modifications ;” and if, among these modifications, the development 
of limbs out of diverging appendages was one intended to charac- 
terize all the higher Vertebrata; then, surely, these diverging ap- 
pendages must have been parts of the “ideal typical vertebra.” 
Or, if the “ ideal typical vertebra” is to be understood as a crystal- 
line form in antagonism with the organizing principle; then why 
should not the appendages be included among its various offshoots ? 
We do not ask this question because of its intrinsic importance. 
We ask it for the purpose of ascertaining Professor Owen’s method 
of determining what are true vertebral constituents. He presents 
us with a diagram of the typical vertebra, in which are included 
certain bones, and from which are excluded certain others. If re- 
lative constancy is the criterion, then there arises the question— 
What degree of constancy entitles a bone to be included? If re- 
lative importance is the criterion, there comes not only the question 
—What degree of importance suffices? but the further question 
—How is importance to be measured? If neither of these is the 
criterion, then what is it? And if there is no criterion, does it 
not follow that the selection is arbitrary ? 


This question serves to introduce a much wider one :—Has the 
‘ideal typical vertebra” any essential constituents at all? It might 


524 


naturally be supposed that though some bones are so rarely 
developed as not to seem worth including, and though some that 
are included are very apt to be absent; yet that certain others are 
invariable: forming, as it were, the basis of the ideal type. Let 
us see whether the facts bear out this supposition. In his “summary 
of modifications of corporal vertebre” (p. 96), Professor Owen 
says— he hamal spine is much less constant as to its existence, 
and is subject to a much greater range of variety, when present, 
than its vertical homotype above, which completes the neural arch.” 
Again he says—“'The hemapophyses, aS osseous elements of a 
vertebra, are less constant than the pleurapophyses.” And again— 
“The pleurapophyses are less constant elements than the neurapo- 
physes.” And again—‘“ Amongst air-breathing vertebrates the 
pleurapophyses of the trunk segments are present only in those species 
in which the septum of the heart’s ventricle is complete and imper- 
forate, and here they are exogenous and confined to the cervical 
and anterior thoracic vertebre.” And once more, both the neura- 
pophyses and the neural spine “are absent under both histological 
conditions, at the end of the tail in most air-breathing vertebrates, 
where the segments are reduced to their central elements.” That 
is to say, of all the peripheral elements of the “ideal typical 
vertebra,” there is not one which is always present. It will be ex- 
pected, however, that at any rate the centrum is constant: the bone 
which “forms the axis of the vertebral column, and commonly the 
central bond of union of the peripheral elements of the vertebra 
(p. 97), is of course an invariable element. No: not even this is 
essential, 

‘‘The centrums do not pass beyond the primitive stage of the notochord 
(undivided column) in the existing lepidosiren, and they retained the like 
rudimental state in every fish whose remains have been found in strata 
earlier than the permian sera in Geology, though the number of vertebra is 


frequently indicated in Devonian and Silurian ichthyolites by the fossilized 
neur- and hemapophyses and their spines” (p. 96). 


Indeed, Professor Owen himself remarks that “the neurapo- 
physes are more constant as osseous or cartilaginous elements of the 
vertebrae than the centrums” (p. 97). Thus, then, it appears that 
the several elements included in the “ideal typical vertebra” have 
various degrees of constancy, and that no one of them is essential. 
There is no one part of a vertebra which invariably answers to its 
exemplar in the pattern-group. How does this fact consist with the 
hypothesis? If the Creator saw fit to make the vertebrate skeleton 
out of a series of segments, all formed on essentially the same model 
— if, for the maintenance of the type, one of these bony segments is 
in many cases formed out of a coalesced group of pieces, where, as 
Professor Owen argues, a single piece would have served as well or 
better ; then we ought to find this typical repetition of parts uni- 


525 


formly manifested. Without any change of shape, it would obvi- 
ously have been quite possible for every actual vertebra to have 
contained all the parts of the ideal one—rudimentally where they 
were not wanted. Even one of the terminal bones of a mammal’s 
tail might have been formed out of the nine autogenous pieces, 
united by suture but admitting of identification. As, however, 
there is no such uniform typical repetition of parts, it seems to us 
that to account for the typical repetition which does occur, by sup- 
posing the Creator to have fixed on a pattern-vertebra, is to ascribe 
to him the inconsistency of forming a plan and then abandoning it. 

If, on the other hand, Professor Owen means that the “ ideal 
typical vertebra” is a crystalline form in antagonism with “ the 
idea or organizing principle ;” then we might fairly expect to find 
it most clearly displaying its crystalline character, and its full com- 
plement of parts, in those places where the organizing principle 
may be presumed to have “subdued” it to the smallest extent. 
Yet in the Vertebrata generally, and even in Professor Owen’s 
Archetypus, the vertebree of the tail, which must be considered as, 
if anything, less under the influence of the organizing principle 
than those of the trunk, do not manifest the ideal form more com- 
pletely. On the contrary, as we approach the end of the tail, the 
successive segments not only lose their remaining typical elements, 
but become as uncrystalline-looking as can be conceived. 


Supposing, however, that the assumption of suppressed or unde- 
veloped elements be granted—supposing it to be consistent with 
the hypothesis of an “ideal typical vertebra,” that the constituent 
parts may severally be absent in greater or less number, sometimes 
leaving only a single bone to represent them all; may it not be that 
such parts as are present, show their respective typical naturcs by 
some constant character: say their mode of ossification ? 

To this question some parts of the Archetype and Homologies seem 
to reply, “ Yes;” while others clearly answer, “No.” Criticising 
the opinions of Geoffroy St. Hilaire and Cuvier, who agreed in 
thinking that ossification from a separate centre was the test of a 
separate bone, and that thus there were as many elementary bones 
in the skeleton as there were centres of ossification, Professor Owen 
points out that, according to this test, the human femur, wnich is 
ossified from four centres, must be regarded as four bones; while 
the femur in birds and reptiles, which is ossified from a single 
centre, must be regarded as a singlebone. Yet, on the other hand, 
he attaches weight to the fact that the skull of the human fetus 
presents ‘“ the same ossific centres” as do those of the embryo kan- 
garoo and the young bird. (Nature of Limbs, p. 40.) And at p. 
104 of the Homologies, after giving a number of instances, he says— 


‘These and the like correspondences between the points of ossification ot 


526 


the human fetal skeleton, and the separate bones of the adult skeletons of 
inferior animals, are pregnant with interest, and rank among the most striking 
illustrations of unity of plan in the vertebrate organization.” 


It is true that on the following page he seeks to explain this 
seeming contradiction by distinguishing 

‘** between those centres.of ossification that have homological relations, and. 
those that have teleological ones—i.¢., between the separate points of ossifica- 
tion of a human bone which typify vertebral elements, often permanently dis- 
tinct bones in the lower animals ; and the separate points which, without such 
signification, facilitate the progress of osteogeny, and have for their obvious 
final cause the well-being of the growing animal.” 


But if there are thus centres of ossification which have homo- 
logical meanings, and others which have not, there arises the ques- 
tion—How are they always to be distinguished? Evidently in 
dependent ossification ceases to be a homological test, if there are 
independent ossifications that have nothing to do with the homo- 
logies. And this becomes the more evident when we learn that 
there are cases where neither a homological nor a teleological 
meaning can be given. Among various modes of ossification of the 
centrum, Professor Owen points out that “the body of the human 
atlas is sometimes ossified from two, rarely from three, distinct 
centres placed side by side” (p. 89); while at p. 87 he says :—“ In 
osseous fishes I find that the centrum is usually ossified from six 
points.” It is clear that this mode of ossification has here no homo- 
logical signification; and it would be difficult to give any teleo- 
logical reason why the small centrum of a fish should have more 
centres of ossification than the large centrum of a mammal. The 
truth is, that as a criterion of the identity or individuality of a bone, 
mode of ossification is quite untrustworthy. Though, in his “ideal 
typical vertebra,” Professor Owen delineates and classifies as sepa- 
rate ‘ autogenous ” elements, those parts which are “ usually 
developed from distinct and independent centres ;” and though by 
doing so he erects this characteristic into some sort of criterion ;. 
yet his own facts show it to be no criterion. The parapophyses. 
are classed among the autogenous elements; yet they are auto- 
venous in fishes alone, and in these only in the trunk vertebra, 
while in all air-breathing vertebrates they are, when present at all, 
exogenous. The neurapophyses, again, ‘“ lose their primitive in-. 
dividuality by various kinds and degrees of confluence:” in the 
tails of the higher Vertebrata they, in common with the neural 
spine, become exogenous. Nay, even the centrum may lose its. 
autogenous character. Describing how, in some batrachians, 
‘“‘ the ossification of the centrum is completed by an extension of 
bone from the bases of the neurapophyses, which effects also the 
coalescence of these with the centrum,” Professor Owen adds :-— 
“In Pelobates fuscus and Pelobates cultripes, Miiller found the en- 


527 


tire centrum ossified from this source, without any independent 
points of ossification” (p. 88). That is to say, the centrum is in 
these cases an exogenous process of the neurapophyses. We see, 
then, that these so-called typical elements of vertebre have no 
constant developmental character by which they can be identified. 
Not only are they undistinguishable by any specific test from other 
bones not included as vertebral elements; not only do they fail to 
show their typical characters by their constant presence; but, 
when present, they exhibit no persistent marks of individuality. 
The central element may be ossified from six, four, three, or two 
points; or it may have no separate point of ossification at all: 
and similarly with various of the peripheral elements. The whole 
group of bones forming the “ ideal typical vertebra” may severally 
have their one or more ossific centres ; or they may, as in a mam- 
mal’s tail, lose their individualities in a single bone ossified from 
one or two points. 


Another fact which seems very difficult to reconcile with the 
hypothesis of an “ideal typical vertebra,” is the not infrequent 
presence of some of the typical elements in duplicate. Not only, 
as we have seen, may they severally be absent ; but they may seve- 
rally be present in greater number than they should be. When we 
see, in the ideal diagram, one centrum, two neurapophyses, two 
pleurapophyses, two hemapophyses, one neural spine, and -one 
hemal spine, we naturally expect to find them always bearing to 
each other these numerical relations. Though we may not be 
greatly surprised by the absence of some of them, we are hardly 
prepared to find others multiplied. Yet such cases are common. 
Thus the neural spine “ is double in the anterior vertebrae of some 
fishes” (p. 98). Again, in the abdominal region of extinct saurians, 
and in crocodiles, “the freely-suspended hemapophyses are com- 
pounded of two or more overlapping bony pieces” (p. 100). Yet 
again, at p. 99, we read—‘ I have observed some of the expanded 
pleurapophyses in the great Testudo elephantopus ossified from two 
centres, and the resulting divisions continuing distinct, but united 
by suture.” Once more “ the neurapophyses, which do not advance 
beyond the cartilaginous stage in the sturgeon, consist in that fish 
of two distinct pieces of cartilage ; and the anterior pleurapophyses 
also consist of two or more cartilages, set end on end” (p. 91). 
And elsewhere referring to this structure, he says :— 

‘* Vegetative repetition of perivertebral parts not only manifests itself in 
the composite neurapophyses and pleurapophyses, but in a small accessory 
(interneural) cartilage, at the fore and back part of the base of the neura- 


pophysis ; and by a similar (interhemal) one at the fore and back part of 
most of the parapophyses” (p. 87). 


Thus the neural and hemal spines, the neurapophyses, the pleu- 


528 


rapophyses, the heemapophyses, may severally consist of two or more 
pieces. This is not all: the like is true even of the centrums. 


‘*In Heptanchus (Squalus cinereus) the vertebral centres are feebly and 
vegetatively marked out by numerous slender rings of hard cartilage in the 
notochordal capsule, the number of vertebre being more definitely indicated 
by the neurapophyses and parapophyses. . . . In the piked dog-fish 
(Acanthias) and the spotted dog-fish (Scyldium) the vertebral centres coin- 
cide in number with-the neural arches ” (p. 87). 


Is it not strange that the pattern vertebra should be so little ad- 
hered to, that each of its smgle typical pieces may be transformed 
into two or three ? 

But there are still more startling departures from the alleged 
type. The numerical relations of the elements vary not only in 
this way, but in the opposite way. A given part may be present 
not only in greater number than it should be, but also in less. In 
the tails of homocercal fishes, the centrums “ are rendered by cen- 
tripetal shortening and bony confluence fewer in number than the 
persistent, neural, and hemal arches of that part ”—that is, there 
is only a fraction of a centrum to each vertebra. Nay, even this 
is not the most heteroclite structure. Paradoxical as it may seem, 
there are cases in which the same vertebral element is, considered 
under different aspects, at once in excess and defect. Speaking of 
the heemal spine, Professor Owen says :— 


‘¢ The horizontal extension of this vertebral element is sometimes accom- 
panied by a median division, or in other words, it is ossified from two 
lateral centres ; this is seen in the development of parts of the human 
sternum ; the same vegetative character is constant in the broader thoracic 
hemal spines of birds ; though, sometimes, as ¢.g., in the struthionide, 
ossification extends from the same lateral centre lengthwise—i.e., forwards and 
backwards, calcifying the connate cartilaginous homologues of halves of four 
or jive hemal spines, before these finally coalesce with their fellows at the 
median line” (p. 101). 

So that the sternum of the ostrich, which according to the hypo- 
thesis, should, in its cartilagmous stage, have consisted of four or 
five transverse pieces, answering to the vertebral segments, and 
should have been ossified from four or five centres, one to each 
cartilaginous piece, shows not a trace of this structure; but in- 
stead, consists of two longitudinal pieces of cartilage, each ossified 
from one centre, and finally coalescing on the median line. These 
four or five hemal spines have at the same time doubled their in- 
dividualities transversely, and entirely lost them longitudinally ! 


There still remains to be considered the test of relative position. . 
It might be held that, sp:te of all the foregoing anomalies, if the 
typical parts of the vertebre always stood towards each other in 
the same reiations——always preserved the same connexions, some- 
thing like a case would be made out. Doubtless, relative position 


529 


is an important point ; and it is one on which Professor Owen mam- 
festly places great dependence. In his discussion of “ moot cases 
of special homology,” it is the general test to which he appeals. 
The typical natures of the alisphenoid, the mastoid, the orbito- 
sphenoid, the prefrontal, the malar, the squamosal, &c., he deter- 
mines almost wholly by reference to the adjacent nerve-perforations 
and the articulations with neighbouring bones (see pp. 19 to 72): 
the general form of the argument being—This bone is to be classed 
as such or such, because it is connected thus and thus with these 
others, which are so and so. Moreover, by putting forth an “ ideal 
typical vertebra,” consisting of a number of elements standing 
towards each other in certain definite arrangement, this persistency - 
of relative position is manifestly alleged. The essential attribute 
of this group of bones, considered as a typical group, is the con- 
stancy in the connexions of its parts: change the connexions, and 
the type is changed. But the constancy of relative position thus 
tacitly asserted, and appealed to as a conclusive test in “ moot 
cases of special homology,” is clearly negatived by Professor 
Owen’s own facts. Jor instance, in the “ideal typical vertebra,” 
the hsemal arch is represented as formed by the two hxemapophyses 
and the hemal spine; but at p. 91 we are told that 


‘The contracted hemal arch in the caudal region of the body may be 
‘formed by different elements of the typical vertebra: e.g., by the para- 
pophyses (fishes generally) ; by the pleurapophyses (lepidosiren) ; by both 
parapophyses and pleurapophyses (Sudis, Lepidosteus), and by hemapo- 
physes, shortened and directly articulated with the centrums (reptiles and 
mammals).”’ 


And further, in the thorax of reptiles, birds, and mammals, “the 
heemapophyses are removed from the centrum, and are articulated t« 
the distal ends of the pleurapophyses; the bony hoop being com 
pleted by the intercalation of the hemal spine” (p. 82). So that 
there are five different ways in which the hemal arch may be formed 
—four modes of attachment of the parts different from that shown 
in the typical diagram! Nor is this all. The pleurapophyses “ may 
be quite detached from their proper segment, and suspended to the - 
hemal arch of another vertebra ;” as we have already seen, the 
entire hemal arch may be detached and removed to a distance, 
sometimes reaching the length of twenty-seven vertebree ; and, even 
more remarkable, the ventral fins of some fishes, which theoretically 
belong to the pelvic arch, are so much advanced forward as to be 
articulated to the scapular arch—“ the ischium elongating to join 
the coracoid.” With these admissions it seems to us that relative 
position and connexions cannot be appealed to as tests of homology, 
nor as evidence of any original type of vertebra. 

In no class of facts, then, do we find a good foundation for the 
hypothesis of an “ideal typical vertebra.” There is no one con- 

VOL, IL 34 


530 


ceivable attribute of this archetypal form which is habitually realised 
by actual vertebree. The alleged group of true vertebral elements 
is not distinguished in any specified way from bones not included in 
it. Its members have various degrees of inconstancy; are rarely 
all present together; and no one of them is essential. They are 
severally developed in no uniform way: each of them may arise 
either out of a separate piece of cartilage, or out of a piece con- 
tinuous with that of some other element; and each may be ossified 
from many independent points, from one, or from none. Not only 
may their respective individualities be lost by absence, or by con- 
fluence with others; but they may be doubled, or tripled, or halved, 
or may be multiplied in one direction and lost in another. The en- 
tire group of typical elements may coalesce into one simple bone 
representing the whole vertebra; and even, as in the terminal piece 
of a bird’s tail, half-a-dozen vertebra, with all their many elements, 
may become entirely lost in a single mass. Lastly, the respective 
elements, when present, have no fixity of relative position: sundry 
of them are found articulated to various others than those with 
which they are typically connected; they are frequently displaced 
and attached to neighbouring vertebre ; and they are even removed 
to quite remote parts of the skeleton. It seems to us that if this 
want of congruity with the facts does not disprove the hypothesis, 
no such hypothesis admits of disproof. 


Unsatisfactory as is the evidence in the case of the trunk and 
tail vertebree, to which we have hitherto confined ourselves, it is far 
worse in the case of the alleged cranial vertebree. The mere fact 
that those who have contended for the vertebrate structure of the 
skull, have differed so astonishingly in their special interpretations 
of it, is enough to warrant great doubt as to the general truth of 
their theory. From Professor Owen's history of the doctrine of 
general homology, we gather that Duméril wrote upon “la téte 
considérée comme une vertebre;” that Kielmeyer, ‘instead of 
calling the skull a vertebra, said each vertebra might be called a 
skull;” that Oken recognized in the skull three vertebre and a 
rudiment ; that Professor Owen himself makes out four vertebre ; 
that Goethe’s idea, adopted and developed by Carus, was, that the 
skull is composed of sia vertebree; and that Geoffroy St. Hilaire 
divided it into seven. Does not the fact that different comparative 
anatomists have arranged the same group of bones into one, three, 
four, six, and seven vertebral segments, show that the mode of de- 
termination is arbitrary, and the conclusions arrived at fanciful 2 
May we not properly entertain great doubts as to any one scheme 
being more valid than the others? And if out of these conflicting 
schemes we are asked to accept one, ought we not to accept it only 
on the production of some thoroughly conclusive proof—some 


d31 


rigorous test showing irrefragably that the others must ne wrong 
and this alone right? Evidently where such contradictory opinions 
have been formed by so many competent judges, we ought, before 
deciding in favour of one of them, to have a clearness of demon- 
stration much exceeding that required in any ordinary case. Let 
us see whether Professor Owen supplies us with any such clearness 
of demonstration. 

To bring the first or occipital segment of the skull into corre- 
spondence with the “ ideal typical vertebra,” Professor Owen argues, 
in the case of the fish, that the parapophyses are displaced, and 
wedged between the neurapophyses and the neural spine—removed 
from the hema! arch and built into the upper part of the neural 
arch. Further, he considers that the pleurapophyses are teleologi- 
cally compound. And then, in all the higher vertebrata, he alleges 
that the hemal arch is separated from its centrum, taken to a dis- 
tance, and transformed into the scapular arch. Add to which, he 
says that in mammals the displaced parapophyses are mere processes 
of the neurapophyses (p. 183): these vertebral elements, typically 
belonging to the lower part of the centrum, and in nearly all cases 
confluent with it, are not only removed to the far ends of elements 
placed above the centrum, but have become exogenous parts of them ! 

Conformity of the second or parietal segment of the cranium with 
the pattern-vertebra, is produced thus :—The petrosals are excluded 
as being partially-ossified sense-capsules, not forming parts of the 
true vertebral system, but belonging to the “ splanchno-skeleton.” 
. A centrum is artificially obtained by sawing in two the bone which 
serves in common as centrum to this and the preceding segment ; and 
this though it is admitted that in fishes, where their individualities 
ought to be best seen, these two hypothetical centrums are not 
simply coalescent, but connate. Next, a similar arbitrary bisection 
is made of certain elements of the hemal arches. And then, “ the 
principle of vegetative repetition is still more manifest in this arch 
than in the occipital one:” each pleurapophysis is double; each 
hemapophysis is double; and the hemal spine consists of six pieces! 

The interpretation of the third and fourth segments being of the 
same general character, need not be detailed. The only point 
calling for remark being, that in addition to the above various 
modes of getting over anomalies, we find certain bones referred to 
the dermo-skeleton. 

Now it seems to us, that even supposing no antagonist interpre- 
tations had been given, an hypothesis reconcilable with the facts 
only by the aid of so many questionable devices, could not be con- 
sidered satisfactory ; and that when, as in this case, various com- 
parative anatomists have contended for other interpretations, the 
character of this one is certainly not of a kind to warrant the re- 
jection of the others in its favour; but rather of a kind to make 

34 * 


532 


us doubt the possibility of all such interpretations. The question 
which naturally arises is, whether by proceeding after this fashion, 
groups of bones might not be arranged into endless typical forms. 
If, when a given element was not in its place, we were at liberty to 
consider it as suppressed, or connate with some neighbouring element, 
or removed to some more or less distant position ;—if, on finding a 
bone in excess, we might consider it, now as part of the dermo- 
skeleton, now as part of the splanchno-skeleton, now as transplanted 
from its typical position, now as resulting from vegetative repetition, 
and now as a bone teleologically compound (for these last two are 
intrinsically different, though often used by Professor Owen as 
equivalents) ;—if, in other cases, a bone might be regarded as 
spurious (p. 91), or again as having usurped the place of another ;— 
if, we say, these various liberties were allowed us, we should not 
despair of reconciling the facts with various diagrammatic types 
besides that adopted by Professor Owen. 

When, in 1851, we attended a course of Professor Owen’s lectures 
on Comparative Osteology, beginning though we did in the attitude 
of discipleship, our scepticism grew as we listened, and reached its 
climax when we came to the skull; the reduction of which to the 
vertebrate structure, reminded us very much of the interpretation 
of prophecy. The delivery, at the Royal Society, of the Croonian 
Lecture for 1858, in which Professor Huxley, confirming the state- 
ments of several German anatomists, has shown that the facts of 
embryology do not countenance Professor Owen’s views respecting 
the formation of the cranium, has induced us to reconsider the verte- 
bral theory as a whole. Closer examination of Professor Owen’s 
doctrines, as set forth in his works, has certainly not removed the 
scepticism generated years ago by his lectures. On the contrary, 
that scepticism has deepened into disbelief. And we venture to think 
that the evidence above cited shows this disbelief to be warranted. 


There remains the question—What general views are we to take 
respecting the vertebrate structure? If the hypothesis of an “ideal 
typical vertebra” is not justified by the facts, how are we to under- 
stand that degree of similarity which vertebre display ? 

We believe the explanation is not far to seek. All that our space 
will here allow, is a brief indication of what seems to us the natural 
view of the matter. 

Professor Owen, in common with other comparative anatomists, 
regards the divergences of individual vertebre from the average 
form, as due to adaptive modifications. If here one vertebral ele- 
ment is largely developed, while elsewhere it is small—if now the 
form, now the position, now the degree of coalescence, of a given 
part varies; it is that the local requirements have involved this 
change. The entire teaching of comparative osteology implies that 


533 


differences in the conditions of the respective vertebree ‘necessitate 
differences in their structures. 

Now, it seems to us that the first step towards a right conception 
of the phenomena, is to recognize this general law in its converse 
application. Ji vertebree are unlike in proportion to the unlikeness 
of their circumstances, then, by implication, they will be like in pro- 
portion to the likeness of their circumstances. While successive 
segments of the same skeleton, and of different skeletons, are all in 
some respects more or less differently acted on by incident forces, 
and are therefore required to be more or less different; they are 
all, in other respects, similarly acted on by incident forces, and are 
therefore required to be more or less similar. It is impossible to. 
deny that if differences in the mechanical functions of the vertebre 
involve differences in their forms; then, community in their mechani- 
cal functions, must involve community in their forms. And as we 
know that throughout the Vertebrata generally, and in each vertebrate 
animal, the vertebre, amid all their varying circumstances, have a 
certain community of function, it follows necessarily that they will 
have a certain general resemblance—there will recur that average 
shape which has suggested the notion of a pattern vertebra. 

A glance at the facts at once shows their harmony with this 
conclusien. In an eel or a snake, where the bodily actions are such 
as to involve great homogeneity in the mechanical conditions of the 
vertebrae, the series of them is comparatively homogeneous. On the 
contrary, in a mammal or a bird, where there is considerable hetero- 
geneity in their circumstances, their similarity is no longer so great. 
And if, instead of comparing the vertebral columns of different 
animals, we compare the successive vertebre of any one animal, we 
recognize the same law. In the segments of an individual spine, 
where is there the greatest divergence from the common mechanical 
conditions ? and where may we therefore expect to find the widest 
departure from the average form? Obviously at the two extremities. 
And accordingly it is at the two extremities that the ordinary struc- 
ture is lost. . 

Still clearer becomes the truth of this view, when we consider the 
genesis of the vertebral column as displayed throughout the ascend- 
ing grades of the Vertebrata. In its first embryonic stage, the spine 
is an undivided column of flexible substance. In the early fishes, 
while some of the peripheral elements of the vertebrae were marked 
out, the central axis was still a continuous unossified cord. And 
thus we have good reason for thinking that in the primitive verte- 
brate animal, as in the existing Amphioxus, the notochord was per- 
sistent. The production of a higher, more powerful, more active 
creature of the same type, by whatever method it is conceived to 
have taken place, involved a change in the notochordal structure. 
Greater muscular endowments presupposed a Srmer internal fulcrum 


534 


—a less yielding central axis. On the other hand, for the central 
axis to have become firmer while remaining continuous, would have 
entailed a stiffness incompatible with the creatures movements. 
Hence, increasing density of the central axis necessarily went hand 
in hand with its segmentation: for strength, ossification was re- 
quired ; for flexibility, division into parts. The production of ver- 
tebre resulting thus, there obviously would arise among them a 
general likeness, due to the similarity in their mechanical conditions, 
and more especially the muscular forces bearing on them. And then 
observe, lastly, that where, as in the head, the terminal position and 
the less space for development of muscles, entailed smaller lateral 
bendings, the segmentation would naturally be less decided, less 
regular, and would be lost as we approached the front of the 
head. 

But, it may be replied, this hypothesis does not explain all the 
facts. It does not tell us why a bone whose function in a given 
animal requires it to be solid, is formed not of a single piece, but by 
the coalescence of several pieces, which in other creatures are sepa- 
rate ; it does not account for the frequent manifestations of unity of 
plan in defiance of teleological requirements. This is quite true. 
But it is not true, as Professor Owen argues respecting such cases, 
that “if the principle of special adaptation fails to explain them, and 
we reject the idea that these correspondences are manifestations of 
some archetypal exemplar, on which it has pleased the Creator to 
frame certain of his living creatures, there remains only the alterna- 
tive that the organic atoms have concurred fortuitously to produce such 
harmony.” ‘This is not the only alternative: there is another, which 
Professor Owen has overicoked. It is a perfectly tenable supposi- 
tion that all higher vertebrate forms have arisen by the superposing of 
adaptations upon adaptations. Hither of the two antagonist cosmo- 
gonies consists with this supposition. If, on the one hand, we con- 
ceive species to have resulted from acts of special creation; then it 
is quite a fair assumption that to produce a higher vertebrate animal, 
the Creator did not begin afresh, but took a lower vertebrate animal, 
and so far modified its pre-existing parts as to fit them for the new 
requirements ; in which case the original structure would show itself 
through the superposed modifications. If, on the other hand, 
we conceive species to have resulted by gradual differentiations 
under the influence of changed conditions; then, it would mani- 
festly follow that the higher, heterogeneous forms, would bear 
traces of the lower and more homogeneous forms from which they 
were evolved. 

Thus, besides finding that the hypothesis of an “ideal typical 
vertebra” is irreconcilable with the facts, we find that the facts are 
interpretable without gratuitous assumptions. The average com- 
munity of form which vertebree display, is explicable as resulting 


535 


from natural causes. And those typical similarities which are trace- 

able under adaptive modifications, must obviously exist if, through- 
out creation in general, there has gone on that continuous super- 
posing of modifications upon modifications which goes on in every 
unfolding organism. 


[I might with propriety have added to the foregoing criticisms, 
the remark that Professor Owen has indirectly conferred a great 
benefit by the elaborate investigations he has made with the view of 
establishing his hypothesis. He has himself very conclusively proved 
that the teleological interpretation is quite irreconcilable with the 
facts. In gathering together evidence in support of his own con- 
ception of archetypal forms, he has disclosed adverse evidence which 
I think shows his conception to be untenable. The result is that 
the field is left clear for the hypothesis of Evolution as the only 
tenable one. ] 


APPENDIX 6. 


[From the TRANSACTIONS OF THE LINNEAN SocIETY, vol. XXV.] 


XV. On Circulation and the Formation of Wood in Plants. By 
Hersert Spencer, Esg. Communicated by Grorce Busk, 
Esq., F.RS., Sec. LS. 


Read March Ist, 1866. 


Oprrnions respecting the functions of the vascular tissues in plants 
appear to make but little progress towards agreement. The suppo- 
sition that these vessels and strings of partially-united cells, lined 
with spiral, annular, reticulated, or other frameworks, are carriers 
of the plant-juices, is objected to on the ground that they often 
contain air: as the presence of air arrests the movement of blood 
through arteries and veins, its presence in the ducts of stems and 
petioles is assumed to unfit them as channels for sap. On the 
other hand, that these structures have a respiratory office, as some 
have thought, is certainly not more tenable, since, if the presence 
of air in thom negatives the belief that their function is to dis- 
tribute liquid, the presence of liquid in them equally negatives the 
belief that their function is to distribute air. Nor can any better 
defence be made for the hypothesis which I find propounded, that 
these parts serve “to give strength to the parenchyma.” ‘Tubes 
with fenestrated and reticulated internal skeletons have, indeed, 
some power of supporting the tissue through which they pass; but 
tubes lined with spiral threads can yield extremely little support, 
while tubes lined with annuli, or spirals alternating with annuli, can 
yield no support whatever. Though all these types of internal 
framework are more or less efficient for preventing closure by 
lateral pressure, they are some of them quite useless for holding 
up the mass through which the vessels pass; and the best of them 
are for this purpose mechanically inferior to the simple cylinder. 
The same quantity of matter made into a continuous tube would be 
more effective in giving stiffness to the cellular tissue around it. 

In the absence of any feasible alternative, the hypothesis that 
these vessels are distributors of sap claims reconsideration. The 
objections are not, I think, so serious as they seem. The habitual 


537 


presence of air in the ducts that traverse wood, can scarcely be ° 
held anomalous if when the wood is formed their function ceases. 
The canals which ramify through a Stag’s horn, contain air after 
the Stag’s horn is fully developed; but it is not thereby rendered 
doubtful whether it is the function of arteries to convey blood. 
Again, that air should frequently be found even in the vessels of 
petioles and leaves, will not appear remarkable when we call to 
mind the conditions to which a leaf is subject. Hvaporation is 
going on from it. The thinner liquids pass by osmose out of the 
vessels into the tissues containing the liquids thickened by evapora- 
tion. And as the vessels are thus continually drained, a draught is 
made upon the liquid contained in the stem and roots. Suppose 
that this draught is unusually great, or suppose that around the 
roots there exists no adequate supply of moisture. <A state of 
capillary tension must result—a tendency of the liquid to pass into 
the leaves resisted below by liquid cohesion. Now, had the vessels 
impermeable coats, only their upper extremities would under these 
conditions be slowly emptied. But their coats, in common with all 
the surrounding tissues, are permeable by air. Hence, under this 
state of capillary tension, air will enter ; and as the upper ends of 
the tubes, being both smaller in diameter and less porous than the 
lower, will retain the liquids with greater tenacity, the air will 
enter the wider and more porous tubes below—the ducts of the 
stem and branches. Thus the entrance of air no more proves that 
these ducts are not sap-carriers, than does the emptiness of tropical 
river-beds in the dry season prove that they are not channels for 
water. There is, however, a difficulty which seems more serious. 
It is said that air, when present in these minute canals, must be a 
great obstacle to the movement of sap through them. ‘The investi- 
gations of Jamin have shown that bubbles in a capillary tube resist 
the passage of liquid, and that their resistance becomes very great 
when the bubbles are numerous—reaching, in some experiments, as 
much as three atmospheres. Nevertheless the inference that any 
such resistance is offered by the air-bubbles in the vessels of a 
plant, is, I think, an erroneous one. What happens in a capillary 
tube having impervious sides, with which these experiments were 
made, will by no meaus happen in a capillary tube having pervious 
sides. Any pressure brought to bear on the column of liquid con- 
tained in the porous duct of a plant, must quickly cause the expul- 
sion of a contained air-bubble through the minute openings in the 
coats of the duct. The greater molecular mobility of gases than 
liquids, implies that air will pass out far more readily than sap. 
Whilst, therefore, a slight tension on the column of sap will cause 
it to part and the air to enter, a slight pressure upon it will force 
out the air and reunite the divided parts of the column. 

To obtain data for an opinion on this vexed question, I have 


538 


lately been experimenting on the absorption of dyes by plants. So 
far as I can learn, experiments of this kind have most, if not all of 
them, been made on stems, and, as it would seem from the results, 
on stems so far developed as to contain all their characteristic 
structures. The first experiments I made myself were on such 
parts, and yielded evidence that served but little to elucidate 
matters. It was only after trying like experiments with leaves of 
different ages and different characters, and with undeveloped axes, 
as well as with axes of special kinds, that comprehensible results 
were reached ; and it then became manifest that the appearances 
presented by ordinary stems when thus tested, are in a great degree 
misleading. Let me briefly indicate the differences. 

If an adult shoot of a tree or shrub be cut off, and have its lower 
end placed in an alumed decoction of logwood or a dilute solution 
of magenta,* the dye will, in the course of a few hours, ascend to a 
distance varying according to the rate of evaporation from the 
leaves. On making longitudinal sections of the part traversed by 
it, the dye is found to have penetrated extensive tracts of the 
woody tissue ; and on making transverse sections, the openings of 
the ducts appear as empty spaces in the midst of a deeply-coloured 
prosenchyma. It would thus seem that the liquid is carried up the 
denser parts of the vascular bundles; neglecting the cambium layer, 
neglecting the central pith, and neglecting the spiral vessels of the 
medullary sheath. Avparently the substance of the wood has 
afforded the readiest channel. When, however, we examine these 
appearances critically, we find reasons for doubting this conclusion. 
If a transverse section of the lower part, into which the dye passed 
first and has remained longest, be compared with a transverse sec- 
tion of the part which the dye has but just reached, a marked 
difference is visible. In the one case the whole of the dense tissue 
is stained; in the other case it is not. This uneven distribution of 
stain in the part which the dye has incompletely permeated is not 
at random ; it admits of definite description. A tolerably regular 
continuous ring of colour distinguishes the outer part of the wood 
from the inner mass, implying a passage of liquid up the elongated 
cells next the cambium layer. And the inner mass is coloured more 
round the mouths of the pitted ducts than elsewhere: the dense 
tissue is darkest close to the edges of these ducts; the colour fades 
away gradually on receding from their edges; there is most colour 
where there are several ducts together ; and the dense tissue which 


* These two dyes have affinities for different components of the tissues, 
and may be advantageously used in different cases. Magenta is rapidly 
taken up by woody matter and other secondary deposits ; while logwood 
colours the cell-membranes, and takes but reluctantly to the substances 
seized by magenta. By trying both of them on the same structure, we may 
guard ourselves against any error arising from selective combination, 


539 


is fully dyed for some space, is that which lies between two or more 
ducts. These are indications that while the layer of pitted cells 
next the cambium has served as a channel for part of the liquid, the 
rest has ascended the pitted ducts. and oozed out of these into the 
prosenchyma around. And this conclusion is confirmed by the 
contrast between the appearances of the lowest part of a shoot 
under different conditions. For if, instead of allowing the dye 
time for oozing through the prosenchyma, the end of the shoot be 
just dipped into the dye and taken out again, we find, on making 
transverse sections of the part into which the dye has been rapidly 
taken up, that, though it has diffused to some distance round the 
, ducts, it has left tracts of wood between the ducts uncoloured—a 
difference which would not exist had the ascent been through the 
substance of the wood. ven still stronger is the confirmation 
obtained by using one dye after another. If a shoot that has ab- 
sorbed magenta for an hour be placed for five minutes in the log- 
wood decoction, transverse sections of it taken at a short distance 
from its end show the mouths of the ducts surrounded by dark 
stains in the midst of the much wider red stains, 

Based on these comparisons only, the inference pointed out has 
little weight; but its weight is increased by the results of experi- 
ments on quite young shoots, and shoots that develope very little 
wood. ‘The behaviour of these corresponds perfectly with the ex- 
pectation that a liquid will ascend capillary tubes in preference to 
simple cellular tissue or tissue not differentiated into continuous canals. 
The vascular bundles of the medullary sheath are here the only 
channels which the coloured liquid takes. In sections of the parts 
up to which the dye has but just reached, the spiral, fenestrated, 
scalariform, or other vessels contained in these bundles are alone 
coloured ; and lower down it is only after some hours that such ar 
exudation of dye takes place as suffices partially to colour the other 
substances of the bundle. Further, it is to be noted that at the 
terminations of shoots, where the vessels are but incompletely formed 
out of irregularly-joined fibrous cells which still retain their original 
shapes, the dye runs up the incipient vessels and does not colour in 
the smallest degree the surrounding tissue. 

Experiments with leaves bring out parallel facts. On placing in 
a dye a petiole of an adult leaf of a tree, and putting it before the 
fire to accelate evaporation, the dye will be found to ascend the 
midrib and veins at various rates, up even to a foot per hour. At 
first it is confined to the vessels; but by the time it has reached the 
point of the leaf, it will commonly be seen that at the lower part it 
has diffused itself into the sheaths of the vessels. In a quite young 
leaf from the same shoot, we find a much more rigorous restriction 
of the dye to the vessels. On making oblique sections of its petiole, 
midrib, and veins, the vessels have the appearance of groups of 


540 


sharply defined coloured rods imbedded in the green prosenchyma ; 
and this marked contrast continues with scarcely an appreciable 
change after plenty of time has been allowed for exudation. 

The facts thus grouped and thus contrasted seem, at first sight, 
to imply that while they are young the coats of these ramifying 
canals lined with spiral or allied structures are not readily perme- 
able, but that, becoming porous as they grow old, they allow the 
liquids they carry to escape with increasing facility ; and hence a 
‘ possible interpretation of the fact that, in the older parts, the stain- 
ing of the tissue around the vessels is so rapid as to suggest that the 
dye has ascended directly through this tissue, whereas in the younger 
parts the reverse appearance necessitates the reverse conclusion. 
But now, is this difference determined by difference of age, or is it 
otherwise determined? The evidence as presented in ordinary stems 
and leaves shows us that the parts of the vascular system at which 
there is a rapid escape of dye are not simply older parts, but are 
parts where a deposit of woody matter is taking place. Is it, then, 
that the increasing permeability of the ducts, imstead of being 
directly associated with their increasing age, is directly associated 
with the increasing deposit of dense substance around them ? 

To get proof that this last connexion is the true one, we have 
but to take a class of cases in which wood is formed only to a small 
extent. In such cases experiments show us a far more general and 
continued limitation of the dye to the vessels. Ordinary herbs and 
vegetables, when contrasted with shrubs and trees, illustrate this; as 
instance the petioles of Celery, or of the common Dock, and the 
ieaves of Cabbages or Turnips. And then in very succulent plants, 
such as Bryophyllum calycinum, Kalanchoé rotundifolia, the various 
species of Crassula, Cotyledon, Kleinia, and others of like habit, the 
ducts of old and young leaves alike retain the dye very persistently : 
the concomitant in these cases being the small amount of prosen- 
chyma around the ducts, or the small amount of deposit in it, or 
both. More conclusive yet is the evidence which meets us when we 
turn from very succulent leaves to very succulent axes. The tender 
young shoots of Klecnia ante-euphorbium, or HKuphorbia Mauritanica, 
which for many inches of their lengths have scarcely any ligneous 
fibres, show us scarcely any escape of the coloured liquid from the 
vessels of the medullary sheath. So, too, is it with Stapelia 
Buffonia, a plant of another order, having soft swollen axes. And 
then we have a repetition of the like connexion of facts throughout 
the Cactacee: the most succulent showing us the smallest perme- 
ability of the vessels. In two species of /hipsalis, in two species of 
Cereus, and in two species of Mammillaria, which I have tried, I 
have found this so. Mammillaria gracilis may be named as ex- 
emplifying the relation under its extreme form. Into one of these 
small spheroidal masses, the dye ascends through the large bundles 


541 


of spiral or annular ducts, or cells partially united into such ducts, 
eolouring them deeply, and leaving the feebly-marked sheath of 
‘prosenchyma, together with the surrounding watery cellular tissue, 
perfectly uncoloured. 

The most conclusive evidence, however, is furnished by those 
‘Cactacee in which the transition from succulent to dense tissue 
takes place variably, according as local circumstances determine. 
Opuntia yields good examples. If a piece of it including one of 
the joints at which wood is beginning to form, be allowed to absorb 
a coloured liquid, the liquid, running up the irregular bundles of 
vessels and into many of their minute ramifications, is restricted to 
these where they pass through the parenchyma forming the mass of 
the stem; but near the joints the hardened tissue around the vessels 
is coloured. In one of these fleshy growths we get clear evidence 
that the escape of the dye has no immediate dependence on the age 
of the vessels, since, in parts of the stem that are alike in age, some 
of the vessels retain their contents while others do not. Nay, we 
even find that the younger vessels are more pervious than the older 
ones, if round the younger ones there is a formation of wood. 

Thus, then, is confirmed the inference before drawn, that in ordi- 
nary stems the staining of the wood by an ascending coloured liquid 
is due, not to the passage of the coloured liquid up the substance 
of the wood, but to the permeability of its ducts and such of its 
pitted cells as are united into irregular canals. And the facts 
showing this, at the same indicate with tolerable clearness the process 
by which wood is formed. What in these cases is seen to take place 
with a dye, may be fairly presumed to take place with sap. Where 
the dye exudes but slowly, we may infer that the sap exudes but 
slowly ; and it is a fair inference that where the dye leaks rapidly out 
of the vessels, the sap does the same. Inferring, thus, that where- 
ever there is a considerable formation of wood there is a considerable 
escape of the sap, we see in the one the result of the other. The 
thickening of the prosenchyma is proportionate to the quantity of 
nutritive liquid passing into it; and this nutritive liquid passes into 
it from the vessels, ducts, and irregular canals it surrounds. 

But an objection is made to such experiments as the foregoing, 
and to all the inferences drawn from them. It is said that portions 
of plants cut off and thus treated, have their physiological actions 
arrested, or so changed as may render the results misleading ; and it 
is said that when detached shoots and leaves have their cut ends 
placed in solutions, the open mouths ot their vessels and ducts are 
directly presented with the liquids to be absorbed, which does not 
happen in their natural states. Further, making these objections 
look serious, it is alleged that when solutions are absorbed through 
the roots, quite different results are obtained: the absorbed matters 
‘are found in the tissues and not in the vessels. Clearly, were the ex- 


542 


periments yielding these adverse results conducted in unobjectionable 
ways, the conclusion implied by them would negative the conclusions 
above drawn. But these experiments are no less objectionable than 
those to which they are opposed. Such mineral matters as salts of iron, 
solutions of which have in some cases been supplied to the roots for 
their absorption, are obviously so unlike the matters ordinarily absorbed, 
that they are likely to interfere fatally with the physiological actions. 
If experiments of this kind are made by immersing the roots in a 
dye, there is, besides the difficulty that the mineral mordant contained 
by the dye is injurious to the plant, the further difficulty that the 
colouring matter, being seized by the substances for which it has an 
affinity, is left behind in the first layers of root tissues passed through, 
and that the decolorized water passing up into the plant is not trace- 
able. To be conclusive, then, an experiment on absorption through 
roots must be made with some solution which will not seriously in- 
terfere with the plant’s vital processes, and which will not have its 
distinctive element left behind. To fulfil these requirements I 
adopted the following method. Having imbedded a well-soaked 
broad-bean in moist sand, contained in an inverted cone of card- 
board with its apex cut off for the radicle to come through—having 
placed this in a wide-mouthed dwarf bottle, partly filled with water, 
so that the protruding radicle dipped into the water—and having 
waited until the young bean had a shoot some three or more inches 
high, and a cluster of secondary rootlets from an inch to an inch 
and a-half long—I supplied for its absorption a simple decoction of 
logwood, which, being a vegetal matter, was not likely to do it much 
harm, and which, being without a mordant, would not leave its sus- 
pended colour in the first tissues passed through. ‘To avoid any 
possible injury, I did not remove the plant from the bottle, but 
slightly raising the cone out of its neck, I poured away the water 
through the crevice and then poured in the logwood decoction; so 
that there could have been no broken end or abraded suriace of a 
rootlet through which the decoction might enter. Being prepared 
with some chloride of tin as a mordant, I cut off, after some three 
hours, one of the lowest leaves, expecting that the application of the 
mordant to the cut surface would bring out the characteristic colour 
if the logwood decoction had risen to that height. I got no re- 
action, however. But after eight hours I found, on cutting off 
another leaf, that the vessels of its petiole were made visible as dark 
streaks by the colour with which they were charged—a colour differ- 
ing, as was to be expected, from that of the logwood decoction, 
which spontaneously changes even by simple exposure. It was then 
too late in the day to pursue the observations ; but next morning the 
vessels of the whole plant, as far as the petioles of its highest, un- 
folded leaves, were full of the colouring-matter; and on applymg 
chloride of tin to the cut suriaces, the vessels assumed that purplish 


543 


red which this mordant produces when directly mixed with the log- 
wood decoction. Subsequently, when one of the cotyledons was cut 
open by Prof. Oliver, to whom, in company with Dr. Hooker, I 
showed the specimen, we found that the whole of its vascular system 
was filled with the decoction, which everywhere gave the characteristic 
reaction. And it became manifest that the liquid absorbed through 
the rootlets, in the central vessels of which it was similarly traceable, 
had part of it passed directly up the vessels of the axis, while part of 
it had passed through other vessels into the cotyledon, out of which, 
no doubt, the liquid ordinarily so carried returns charged with a 
supply of the stored nutriment. I have since obtained a verification 
by varying the method. Digging up some young plants (Marigolds 
happened to afford the best choice) with large masses of soil round 
them, placing them in water, so as gradually to detach the soil with- 
out injuring the rootlets, planting them afresh in a flower-pot full of 
washed sand, and then, after a few days, watering them with a log- 
wood decoction, I found, as before, that in less than twenty-four 
hours the colouring-matter had run up into the vessels of the leaves. 
Though the reaction produced by the mordant was not so strong as 
before, it was marked enough to be quite unquestionable. 

As these experiments were so conducted that there was no access 
to the vessels except through the natural channels, and as the vital 
actions of the plants were so little interfered with that at the end of 
twenty-four hours they showed no traces of disturbance, I think the 
results must be held conclusive. 

Taking it, then, as a fact that in plants possessing them the vessels 
and ducts are the channels through which sap is distributed, we come 
now to the further question—W hat determines the varying permea- 
bility of the walls of the vessels and ducts, and the consequent vary- 
ing formation of wood? ‘To this question I believe the true reply is— 
The exposure of the parts to intermittent mechanical strains, actual 
or potential, or both. By actual strains I of course mean those 
which the plant experiences in the course of its individual life. By 
potential strains I mean those which the form, attitude, and circum- 
stances common to its kind involve, and which its inherited structure 
is adapted to meet. In plants with stems, petioles, and leaves, 
having tolerably constant attitudes, the increasing porosity of the 
tubes and consequent deposit of dense tissue takes place in anticipa- 
tion of the strains to which the parts of the individual are liable, but 
takes place at parts which have been habitually subject to such 
strains in ancestral individuals. But though in such plants the 
tendency to repeat that distribution of dense tissue caused by 
mechanical actions on past generations, goes on irrespective of the 
mechanical actions to which the developing individual is subject, 
these direct actions, while they greatly aid the assumption of the 
typical structure, are the sole causes of those deviations in the rela- 


544 


tive thickenings of parts which distinguish the individual from others 
of its kind. And then, in certain irregularly growing plants, such as 
Cactuses and Euphorbias, where the strains fall on parts that do 
not correspond in successive individuals, we distinctly trace a direct 
relation between the degrees of strain and the rates of these changes 
which result in dense tissue. I will not occupy space in detailing 
the evidence of this relation, which is conspicuous in the orders 
named, but will pass to the question— W hat are the physical processes 
by which intermittent mechanical strams produce this deposit of 
resistant substance at places where it is needed to meet the strains ? 
We have not to seek far for an answer. Ifa trunk, a bough, a 
‘shoot, or a petiole, is bent by a gust of wind, the substance of its 
convex side is subject to longitudinal tension: the substance of its 
concave side being at the same time compressed. This is the 
primary mechanical effect. There is, however, a secondary mechani- 
cal effect, which here chiefly concerns us. That bend by which the 
tissues of the convex side are stretched, also produces lateral com- 
pression of them. Buttoning on a tight glove and then closing the 
hand, will make this necessity clear: the leather, while it is strained 
along the backs of the fingers, presses with considerable force on the 
‘knuckles. It is demonstrable that the tensions of the outer layer of 
@ mass made convex by bending, must, by composition of forces, 
produce at every point a resultant at right angles to the layer be- 
neath it; that, similarly, the joint tensions of these two layers must 
throw a pressure on the next deeper layer; and soon. Hence, if 
at some little distance beneath the surface of a stem, twig, or leaf- 
stalk, there exist longitudinal tubes, these tubes must be squeezed 
each time the side of the branch they are placed on becomes convex. 
Modifying the illustration just drawn from the clenched hand will 
make this clear. When, on forcibly grasping something, the skin is 
drawn tightly over the back of the hand, the whitening of the 
knuckles shows how the blood is expelled from the vessels below the 
surface by the pressure of the tightened skin. If, then, the sap- 
vessels must be thus compressed, what will happen to the liquid they 
contain? It will move away along the lines of least resistance. 
Part, and probably the greater part, will escape lengthways from the 
place of greatest pressure: some of it being expelled downwards, 
and some of it upwards. But, at the same time, part of it will be 
likely to ooze through the walls of the tubes. If these walls are so 
perfect as to permit the passage of liquid only by osmose, it may 
still be inferred that the osmose will increase under pressure; and 
probably, under recurrent pressure, the places at which the osmotic 
-eurrent passes most readily will become more and more permeable, 
until they eventually form pores. At any rate it is manifest that 
‘where pores and slits exist, whether thus formed or formed in any 
other way, the escape of sap into the adjacent tissue at each bend 


545 


will become easy and rapid. What further must happen? When 
the branch or shoot recoils, the vessels on the side that was convex, 
being relieved from pressure, will tend to resume their previous 
diameters ; and will be helped to do this by the elasticity of the sur- 
rounding tissue, as well as by those spiral, annular, and allied struc- 
tures which they contain. But this resumption of their previous 
diameters must cause an immediate rush of sap back into them. 
~ Whence will it come? Not to any considerable extent from the sur- 
rounding tissues into which part of it has been squeezed, seeing that 
the resistance to the return of liquid through small pores will be 
greater than the resistance to its return along the vessels themselves. 
Manifestly the sap which was thrust up and down the vessels from 
the place of compression will return—the quantities returning from 
above and from below varying, as we shall hereafter see, according 
to circumstances. But this is not all. From some sidé a greater 
quantity must come back than was sent away ; for the amount that 
has escaped out of the tube into the prosenchyma has to be replaced. 
Thus during the time when the side of the branch or twig becomes 
concave, more sap returns from above or below than was expelled 
upwards or downwards during the previous compression. The re- 
filled vessels, when the next bend renders their side convex, again 
have part of their contents forced through their parietes, and are 
again refilled in the same way. ‘There is thus set up a draught of sap 
to the place where these intermittent strains are going on, an exuda- 
tion proportionate to the frequency and intensity of the strains, 
and a proportionate nutrition or thickening of the wood- 
cells, fitting them to resist the strains. A rude idea of 
this action may be obtained by grasping in one hand a damp 
sponge, having its lower end in water, while holding a piece of 
blotting-paper in contact with its upper end, and then giving the 
sponge repeated squeezes. At each squeeze some of the water will 
be sent into the blotting-paper; at each relaxation the sponge will 
refill from below, to give another portion of its contents to the 
blotting paper when again squeezed. 

But how does this explanation apply to roots? If the formation 
of wood is due to intermittent transverse strains, such as are pro- 
duced in the aérial parts of upright plants by the wind, how does it 
happen that woody matter is deposited in roots, where there are no 
lateral oscillations, no transverse strains? The answer is, that 
longitudinal strains also are capable of causing the effects described. 
It is true that perfectly straight fibres united into a bundle and pulled 
lengthways would not exert on one another any lateral pressure, and 
would not laterally compress any similarly-straight canals running 
along with them. But if the fibres united into a bundle are variously 
bent or twisted, they cannot be longitudinally strained without com- 
pressing one another and structures imbedded in them. It needs 

VOL. II. 35 


546 


but to watch a wet rope drawn tight by a capstan, to see that an 
action like that which squeezes the water out of its strands, will 
squeeze the sap out of the vessels of a root into the surrounding 
tissue, as often as the root is pulled by the swaying of the plant it 
belongs to. Here, too, as before, the vessels will refill when the 
pull intermits; and so, in the roots as in the branches, this rude 
pumping process will produce a growth of hard tissue proportionate 
to the stress to be borne. 

These conclusions are supported by the evidence which exceptional 
cases supply. If intermittent mechanical strains thus cause the for- 
mation of wood where wood is found, then where it is not found, 
there should be an absence of intermittent mechanical strains. There 
is such an absence. Vascular plants characterized by little or no 
deposit of dense substance, are those having vessels so conditioned 
that no considerable pressures are borne by them. The more 
succulent a petiole or leaf becomes, the more do the effects of trans- 
verse strains fall on its outer layers of cells. Its mechanical support 
is chiefly derived from the ability of these minute vesicles, full of 
liquid, to resist bursting and tearing under the compressions and 
tensions they are exposed to. And just as fast as this change from 
a thin leaf or foot-stalk to a thick one entails increasing stress on the 
superficial tissue, so fast does it diminish the stress on the internally- 
seated vascular tissue. The succulent leaf cannot be swayed about 
by the wind as much as an ordinary leaf; and such small bends as 
can be given to it and its foot-stalk are prevented from affecting in 
any considerable degree the tubes running through its interior. 
Hence the retentiveness of the vessels in these fleshy leaves, as shown 
by the small exudation of dye; and hence the small thickening of 
their surrounding prosenchyma by woody deposit. Still more con- 
spicuously is this connexion of facts shown when, from the soft thick 
leaves before named and such others as those of Hcheveria, Rochea, 
Pereskia, we turn to the thick leaves that have strong exo-skeletons. 
Gasteria serves as an illustration. 'The leathery or horny skin here 
evidently bears the entire weight of the leaf, and is so stiff as to pre- 
vent any oscillation. Here, then, the vessels running inside are pro- 
tected from all mechanical stress; and accordingly we find that the 
cells surrounding them are not appreciably thickened. 

Equally clear, and more striking because more obviously excep- 
tional, is the evidence given by succulent stems which are leafless. 
Stapelia Buffonia, having soft procumbent axes not liable to be bent 
backwards and forwards in any considerable degree by the wind, 
has, ramifying through its tissue, vessels that allow but an extremely 
slow escape of dye and have unthickened sheaths. Such of the 
Kuphorbias as have acquired the fleshy character while retaining the 
arborescent growth, like Huphorbia Canariensis, teach us the same 
truth in another way. In them the formation of wood around the 


547 


vessels is inconspicuous where the intermittent strains are but slight; 
but it is conspicuous at those joints on which lateral oscillations of 
the attached branches throw great extensions and compressions of 
tissue. Throughout the Cactacee we find varied examples of the 
alleged relation. Mammullaria furnishes a very marked one. The 
substance of one of these globular masses, resting on the ground, 
admits of no bending from side to side; and accordingly its large 
bundles of spiral and annular vessels, or partially-united cells, have 
very feebly-marked sheaths not at all thickened. In such types as 
Cereus and Opuntia we see, as in the Huphorbias, that where little 
stress falls on the vessels, little deposit takes place around them; 
while there is much deposit where there is much stress. Here let me 
add a confirmation obtained since writing the above. After observ- 
ing among the Cactuses the very manifest relation between strain 
and the formation of wood, I inquired of Mr. Croucher, the intelli- 
gent foreman of the Cactus-house at Kew, whether he found this 
relation a constant one. He replied that he did, and that he had 
frequently tested it by artificially subjecting parts of them to strains. 
Neglecting at the time to inquire how he had done this, it afterwards 
occurred to me that if he had so done it as to cause constant strains, 
the observed result would not tell in favour of the foregoing inter- 
pretation. Subsequently, however, I learned that he had produced 
the strains by placing the plants in inclined attitudes—a method 
which, by permitting oscillations of the strained joints, allowed the 
strains to intermit. And then, making the proof conclusive, Mr. 
Croucher volunteered the statement that where he had produced 
constant strains by tying, no formation of wood took place. 
Aberrant growths of another class display the same relations of 
phenomena. ‘T'ake first the underground stems, such as the Potato 
and the Artichoke. The vessels which run through these, slowly 
take up the dye without letting it pass to any considerable extent 
into the surrounding tissues.* Only after an interval of many hours 
does the prosenchyma become stained in some places. Here, as 
before, an absence of rapid exudation accompanies an absence of 
woody deposit; and both these go along with the absence of inter- 
mittent strains. Take again the fleshy roots. The Turnip, the 
Carrot, and the Beetroot, have vessels that retain very persistently 
the coloured liquids they take up. And differing in this, as these 
roots do, from ordinary roots, we see that they also differ from them 
in not being woody, and in not being appreciably subject to the 


* Those who repeat these experiments must be prepared for great irregu- 
larities in the rates of absorption. Succulent structures in general absorb 
much more slowly than others, and sometimes will scarcely take up the dye 
at all. The differences between different structures, and the same structure 
at different times, probably depend on the degrees in which the tissues are 
charged with liquid and the rates at which they are losing it by evaporation. 


4 
25 * 


548 


usual mechanical actions. In these cases, as in the others, parts 
that ordinarily become dense, deviate from this typical character 
when they are not exposed to those forces which produce dense 
tissue by increasing the extravasation of sap. 

To complete the proof that such a relation exists, let me add the 
results of some experiments on equal and similarly-developed parts, 
kept respectively at rest and in motion. I have tested the effects on 
large petioles, on herbaceous shoots, and on woody shoots. If two 
such petioles as those of Rhubarb, with their leaves attached, have 
their cut ends inserted in bottles of dye, and the one be bent back- 
wards and forwards while the other remains motionless, there arises, 
after the lapse of an hour, scarcely any difference in the states of 
their vessels : a certain proportion of these are in both cases charged 
with the dye, and little exudation has been produced by the motion. 
Here, however, it is to be observed that the causes of exudation are 
scarcely operative; the vascular bundles are distributed all through 
the mass of the petiole, which is formed of soft watery tissue; and 
they are, therefore, not so circumstanced as to be effectually com- 
pressed by the bends. In herbaceous stems, such as those of the 
Jerusalem Artichoke and of the Foxglove, an effect scarcely more 
decided is produced ; and here, too, when we seek a reason, we find 
it in the non-fulfilment of the mechanical conditions; for the vascular 
bundles are not so seated between a tough layer of bark and a solid 
core as to be compressed at each bend. When, however, we come 
to experiment upon woody shoots, we meet with conspicuous effects, 
though by no means uniformly. In some cases oscillations produce 
immense amounts of exudation—parallel transverse sections of the 
compared shoots showing that where, in the one that has been at 
rest, there are spots of colour round but a few pitted ducts, in the 
one that has been kept in motion the substance of the wood is soaked 
almost uniformly through with dye. In other cases, especially where 
there is much undifferentiated tissue remaining, the exudation is not 
very marked, ‘The difference appears to depend on the quantity of 
liquid contained in the shoot. If its substance is relatively dry, the 
exudation is great; but it is comparatively small if all the tissues are 
fully charged with sap. This contrast of results is one which con- 
templation of the mechanical actions will lead us to expect. 

And now, with these facts to aid our interpretation, let us return 
to ordinary stems. If the upper end of a growing shoot, the prosen- 
chyma of which is but little thickened, be allowed to imbibe the dye, 
the vessels of its medullary sheath alone become charged ; and from 
them there takes place but a slow oozing. If a like experiment be 
tried with a lower part of the shoot, where the wood in course of 
formation has its inner boundary marked but not its outer boundary, 
we find that the pitted ducts, and more especially the inner ones, 
come into play. And then lower still, where the wood has its peri- 


549 


phery defined and its histological characters decided, the appearances 
show that the tissue forming its outer surface begins to take a lead- 
ing part in the transmission of liquid. What now is the explanation 
of these changes, mechanically considered? In the young soft part 
of the shoot, as in all normal and abnormal growths that have not 
formed wood, the channels for the passage of sap are the spiral, 
annular, fenestrated, or reticulated vessels. These vessels, here in- 
cluded in the bundles of the medullary sheath, are, in common with 
the tissues around them, subject, by the bendings of the shoot, to 
slight intermittent compressions, and, especially the outermost of 
them, are thus forced to give the prosenchyma an extra supply of 
nutritive liquid. The thickening of the prosenchyma, spreading 
laterally as well as outwards from each bundle of the medullary 
sheath, goes on until it meets the thickenings that spread from the 
other bundles ; and there is so formed an irregular cylinder of har- 
dened tissue, surrounding the medulla and the vascular bundles of 
its sheath. As soon as this happens, these vascular bundles become, 
to a considerable extent, shielded from the effects of transverse 
strains, since the tensions and compressions chiefly fall on the de- 
veloping wood outside of them. Clearly, too, the greatest stress 
must be felt by the outer layer of the developing wood: being fur- 
ther removed from the neutral axis, it must be subject to severer 
strains at each bend; and lying between the bark and the layer of 
wood first formed, it must be most exposed to lateral compressions. 
Among the elongated cells of this outer layer, some unite to form 
the pitted ducts. Being, as we see, better circumstanced mechani- 
cally, they become greater carriers of sap than the original vessels, 
and, in consequence of this, as well as in consequence of their rela- 
tive proximity, become the sources of nutrition to the still more ex- 
ternal layers of wood-cells. The same causes and the same effects 
hold with each new indurated coat deposited round the previously 
indurated coats. 

This description may be thought to go far towards justifying the 
current views respecting the course taken by the sap. But the 
justification is more apparent than real. In the first place, the im- 
plication here is that the sap-carrying function is at first discharged 
entirely by the vessels of the medullary sheath, and that they cease 
to discharge this function only as fast as they are relatively incapaci- 
tated by their mechanical circumstances. And the second implica- 
tion is, that it is not the wood itself, but the more or less continuous 
canals formed in it, which are the subsequent sap-distributors. This, 
though readily made clear by microscopic examination of the large 
pitted ducts in a partially lignified shoot that has absorbed the dye, 
is less manifestly true of the peripheral layer of sap-carrying tissue 
finally formed. But it is really true here. Lor this layer, though 
nominally a layer of wood, is practically a layer of inosculating 


550 


vessels. It is formed out of irregular lines and networks of elon- 
gated pitted cells, obliquely united by their ends. Hxamination of 
them after absorption of a dye, shows that it is only along the con- 
tinuous channels they unite to form that the current has passed. 
But the essentially vascular character of this outer and latest-formed 
layer of the alburnum is best seen in the fact that the vascular sys- 
tems of new axes take their rise from it, and form .with it continuous 
canals. If a shoot of last year in which growth is recommencing, be 
cut lengthways after it has imbibed a dye, clear proof is obtained 
that the passage of the dye into a lateral bud takes place from this 
outermost layer of pitted cells, and that the channels taken by the 
dye through the new tissue are composed of cells that pass through 
modified forms into the spiral vessels of the new medullary sheath. 
This transition may be still more clearly traced in a terminal bud 
that continues the line of last year’s shoot. A longitudinal section 
of this shows that the vessels of the new medullary sheath do not 
obtain their sap from the vessels of last year’s sheath (which, as 
shown by the non-absorption of dye, have become inactive), but that 
their supplies are obtained from those inosculating canals formed out 
of last year’s outermost layer of prosenchyma, and that between the 
component cells of this and those of the new vascular system there 
are all gradations of structure.* 


* It may be added here that, on considering the mechanical actions that 
must go on, we are enabled in some measure to understand both how such inos- 
culating channels are initiated, and how the structures of their component 
cells are explicable. What must happen to one of these elongated prosen- 
chyma-celis if, in the course of its development, it is subject to intermittent 
compressions? Its squeezed-out liquid while partially escaping laterally, 
will more largely escape upwards and downwards ; and while repeated 
lateral escape will tend to form lateral channels communicating with 
laterally-adjacent cells, repeated longitudinal escape will tend to form 
channels communicating with longitudinally-adjacent cells—so pro- 
ducing continuous though irregular longitudinal canals. Meanwhile 
each cell into and out of which the nutritive liquid is from time 
to time squeezed through small openings in its walls, cannot thicken 
internally in an even manner: deposition will be interfered with by 
the passage of the currents through the pores. The rush to or from each 
pore will tend to maintain a funnel-shaped depression in the deposit around ; 
and the opening from cell to cell will so acquire just that shape which the 
microscope shows up—two hollow cones with their apices meeting at the 
point where the cell-membranes are in contact. Moreover, as confirming 
this interpretation, it may be remarked that we are thus supplied with a 
reason for the differences of shape between these passages from one pitted 
cell to another, and the analogous passages that exist between cells other- 
wise formed and otherwise conditioned. In the cells of the medulla, and 
others which are but little exposed to compression, the passages are seve- 
rally formed more like a tube with two trumpet-mouths, one in each cell. 
This is just the form which might be expected where the nutritive fluid 
passes from cell to cell in moderate currents, and not by the violent rushes 
caused by intermittent pressures. Of course it is not meant that in each 


551 


It is not the aim of the foregoing reasoning to show that mechani 
cal actions are the sole causes of the formation of dense tissue in 
plants. Dense tissue is in many cases formed where no such causes 
have come’ into play—as, for example, in thorns and in the shells of 
nuts. Here the natural selection of variations can alone have ope- 
rated. It is manifest, too, that even those supporting structures the 
building up of which is above ascribed to intermittent strains, may, 
in the individual plant of a species that ordinarily has them, be de- 
veloped to a great extent when intermittent strains are prevented. 
We see this in trees that are artificially supported by nailing to 
walls ; and we also see a kindred fact in natural climbers. Though 
in these cases the formation of wood is obviously less than it would 
be were the stem and branches habitually moved about by the wind, it 
nevertheless goes on. Clearly the tendency of the plant to repeat the 
. structure of its type (in the one case the structure of its species, and in 
the other case that of the order from which it has diverged in becom- 
ing a climber) is here almost the sole cause of wood-formation. But 
though in plants so circumstanced intermittent mechanical strains have 
little or no direct share, it may still be true, and I believe is true, that 
intermittent mechanical strains are the original cause; for, as before 
hinted, the typical structure which the individual thus repeats irre- 
spective of its own conditions, is interpretable as a typical structure 
that is itself the product of these actions and reactions between the 
plant and its environment. Grant the inheritance of functionally- 
produced modifications; grant that natural selection will always co- 
operate in such way as to favour those individuals and families in 
which functionally-produced modifications have progressed most ad- 
vantageously ; and it will follow that this mechanically-caused forma- 
tion of dense substance, accumulating from generation to generation 
by the survival of the fittest, will result in an organic habit of form- 
ing dense tissue at the required places. The deposit arising from 
exudation at the places of greatest strain, recurring from generation 
to generation at the same places, will come to be reproduced in an- 
ticipation of strain, and will continue to be reproduced for a long 
time after a changed habit of the species prevents the strain—even- 
tually, however, decreasing, both through functional inactivity and 
natural selection, to the point at which it is in equilibrium with the 
requirement. 


individual cell these structures are determined by these mechanical actions. 
The facts clearly negative any such conclusion, showing us, as they in many 
cases do, that these structures are assumed in advance of these mechanical 
actions. The implication is, that such mechanical actions initiated modifi- 
cations that have, with the aid of natural selection, been accumulated from 
generation to generation ; until, in conformity with ordinary embryological 
laws, the cells of the parts exposed to such actions assume these special 
structures irrespective of the actions—the actions, however, still serving to 
aid and complete the assumption of the inherited type. 


552 


Another side of the general question may now be considered. We 
have seen how, by intermittent pressures on capillary vessels and 
ducts and inosculating canals, there must be produced a draught of 
sap towards the point of compression to replace the sap squeezed out. 
But we have still to inquire what will be the effect on the distribu- 
tion of sap throughout the plant as a whole. It was concluded that 
out of the compressed vessels the greater part of the liquid would 
escape longitudinally—the longitudinal resistance to movement being 
least. In every case the probabilities are infinity to one against the 
resistances being equal upwards and downwards. Always, then, 
more sap will be expelled in one direction than in the other. But mn 
whichever direction least sap is expelled, from that same direction 
most sap will return when the vessels are relieved from pressure—the 
force which is powerful in arresting the back current in that direction 
being the same force which is powerful. in producing a forward cur- 
rent. Ordinarily, the more abundant supply of liquid being from below, 
there will result an upward current. At each bend a portion of the con- 
tents will be squeezed out through the sides of the vessels—a portion 
will be squeezed downwards, reversing the current ascending from the 
roots. but soon stopped by its resistance ; while a larger portion will 
be squeezed upwards towards the extremities of the vessels, where 
consumption and loss are most rapid. At each recoil the vessels will 
be replenished, chiefly by the repressed upward current; and at the 
next bend more of it will be thrust onwards than backwards. Hence 
we have everywhere in action a kind of rude force-pump, worked by 
the wind; and we see how sap may thus be raised to a height far 
beyond that to which it could be raised by capillary action, aided by 
osmose and evaporation. 

Thus far, however, the argument proceeds on the asumption that 
there is liquid enough to replenish every time the vessels subject 
to this process. But suppose the supply fails—suppose the roots 
have exhausted the surrounding stock of moisture. Evidently the 
vessels thus repeatedly having their contents squeezed out into the 
surrounding tissue, cannot go on refilling themselves from other 
vessels without tending to empty the vascular system. On the one 
hand, evaporation from the leaves causing a draught on the capillary 
tubes that end in them, continually generates a capillary tension up- 
wards ; while, on the other hand, the vessels below, expanding after 
their sap has been squeezed out, produce a tension both upwards 
and downwards towards the point of loss. Were the limiting mem- 
branes of the vessels impermeable, the movement of sap would, under 
these conditions, soon be arrested. But these membranes are perme- 
able; and the surrounding tissues readily permit the passage of air. 
This state of tension, then, will cause an entrance of air into the tubes: 
the columns of liquid they contain will be interrupted by bubbles. 
It seems, indeed, not improbable that this entrance of air may take 


553 


place even when there is a good supply of liquid, if the mechanical 
strains are so violent and the exudation so rapid that the currents 
cannot refil the half-emptied vessels with sufficient rapidity. And in 
this case the intruding air may possibly play the same part as that 
contained in the air-chamber of a force-pump—tending, by moderat- 
ing the violence of the jets, and by equalizing the strains, to prevent 
rupture of the apparatus. Of course when the supply of liquid 
becomes adequate, and the strains not too violent, these bubbles will 
be expelled as readily as they entered. 

Here, as before, let me add the conclusive proof furnished by a 
direct experiment. ‘T'o ascertain the amount of this propulsive 
action, [ took from the same tree, a Laurel, two equal shoots, and 
placing them in the same dye, subjected them to conditions that 
were alike in all respects save that of motion: while one remained 
at rest, the other was bent backwards and forwards, now by switch- 
ing and now by straining with the fingers. After the lapse of an 
hour, I found that the dye had ascended the oscillating shoot three 
times as far as it had ascended the stationary shoot—this result 
being an average from several trials. Similar trials brought out 
similar effects in other structures. 'The various petioles and herba- 
ceous shoots experimented upon for the purpose of ascertaining the 
amount of exudation produced by transverse strains, showed also 
the amount of longitudinal movement. It was observable that the 
height ascended by the dye was in all cases greater where there had 
been oscillation than where there had been rest—the difference, 
however, being much less marked in succulent structures than in 
woody ones. 

It need scarcely be said that this mechanical action is not here 
assigned as the sole cause of circulation, but as a cause co-operating 
with others, and helping others to produce effects that could not 
otherwise be produced. ‘Trees growing in conservatories afford us 
_ abundant proof that sap is raised to considerable heights by other 
forces. Though it is notorious that trees so circumstanced do not 
thrive unless, through open sashes, they are frequently. subject to 
breezes sufficient to make their parts oscillate, yet there is evidently 
a circulation that goes on without mechanical aid. The causes of 
circulation are those actions only which disturb the liquid equilibrium 
in a plant, by permanently abstracting water or sap from some part 
of it; and of these the first is the absorption of materials for the for- 
mation of new tissue in growing parts; the second is the loss by 
evaporation, mainly through adult leaves ; and the third is the loss by 
extravasation, through compressed vessels. Only so far as it pro- 
duces this last, can mechanical strain be regarded as truly a cause of 
circulation. All the other actions concerned must be classed as azds 
to circulation—as facilitating that redistribution of liquid that con- 
tinually restores the equilibrium continually disturbed ; and of these, 


554 


capillary action may be named as the first, osmose as the second, and 
the propulsive effect of mechanical strains as the third. The first 
two of these aids are doubtless capable by themselves of producing a 
large part of the observed result—more of the observed result than is 
at first sight manifest; for there is an important indirect effect of 
osmotic action which appears to be overlooked. Osmose does not 
aid circulation only by setting up, within the plant, exchange currents 
between the more dense and the less dense solutions in different parts 
of it; but it aids circulation much more by producing distention of 
the plant as a whole. In consequence of the average contrast in 
density between the water outside of the plant and the sap inside of it, 
the constant tendency is for the plant to absorb a quantity in excess 
of its capacity, and so to produce distention and erection of its 
tissues. It is because of this that the drooping plant raises itself 
when watered; for capillary action alone could only refill its tissues 
without changing their attitudes. And it is because of this that 
juicy plants with collapsible structures bleed so rapidly when cut, not 
only from the cut surface of the rooted part, but from the cut sur- 
face of the detached part—the elastic tissues tending to press out the 
liquid which distends them. And manifestly if osmose serves thus 
to maintain a state of distention throughout a plant, it indirectly fur- 
thers circulation ; since immediately evaporation or growth at any 
part, by abstracting liquid from the neighbouring tissues, begins to 
diminish the liquid pressure within such tissues, the distended struc- 
tures throughout the rest of the plant thrust their liquid contents to- 
wards the place of diminished pressure. This, indeed, may very pos- 
sibly be the most efficient of the agencies at work. Remembering 
how great is the distention producible by osmotic absorption—great 
enough to burst a bladder—it is clear that the force with which the 
distended tissues of a plant urge forward the sap to places of con- 
sumption, is probably very great. We must therefore regard the aid 
which mechanical strains give as being one of several. Oscillations 
help directly to restore any disturbed liquid equilibrium; and they 
also help indirectly, by facilitating the redistribution caused by capil- 
lary action and the process just described; but in the absence of 
oscillations the equilibrium may still be restored, though less rapidly 
and within narrower limits of distance. 

One half of the problem of the circulation, however, has been left 
out of sight. Thus far our inquiry has been, how the ascending cur- 
rent of sap is produced. ‘There remains the rationale of the descend- 
ing current. What forces cause it, and through what tissues it takes 
place, are questions to which no satisfactory answers have been 
given. That the descent is due to gravitation, as some allege, 
is difficult to conceive, since, as gravitation acts equally on all 
liquid columns contained in the stem, it is not easy to see 
why it should produce downward movements in some while per- 


590 


mitting upward movements in others—uniess, indeed, there existed 
descending tubes too wide to admit of much capillary action, which 
there do not. Moreover, gravitation is clearly inadequate to cause 
currents towards the roots out of branches that droop to the 
ground. Here the gravitation of the contained liquid columns 
must nearly balance that of the connected columns in the stem, 
leaving no appreciable force to cause motion. Nor does there seem 
much probability in the assumption that the route of the descending 
sap is through the cambium layer, since experiments on the absorp- 
tion of dyes prove that simple cellular tissue is a very bad conductor 
of liquids: their movement through it does not take place with one- 
fiftieth of the rapidity with which it takes place through vessels.* 

Of course the defence for these hypotheses is, that there must be a 
downward current, which must have a course and a cause ; and the 
very natural assumption has been that the course and the cause must 
be other than those which produce the ascending current. Never- 
theless there is an alternative supposition, to which the foregoing 
considerations introduce us. It is quite possible for the same vascular 
system to serve as a channel for movement in opposite directions at 
different times. We have among animals well-known cases in 
which the blood-vessels carry a current first in one direction and 
then, after a brief pause, in the reverse direction. And there seems 
an @ priort probability that, lowly organized as they are, plants are 
more likely to have distributing appliances of this imperfect kind than 
to have two sets of channels for two simultaneous currents. If, led 
by this suspicion, we inquire whether among the forces which unite 
to produce movements of sap, there are any variations or inter- 
missions capable of determining the currents in different directions, 
we quickly discover that there are such, and that the hypothesis of 
an alternating motion of the sap, now centrifugal and now centri- 
petal, through the same vessels, has good warrant. What are the 
several forces at work? First may be set down that tendency 
existing in every part of a plant to expand into its typical form, and 
to absorb nutritive liquids in doing this. The resuiting competition 


* Some exceptions to this occur in plants that have retrograded in the 
character of their tissues towards the simpler vegetal types. Certain very 
succulent leaves, such as those of Sempervivum, in which the cellular tissue 
is immensely developed’ in comparison with the vascular tissue, seem to 
have resumed to a considerable extent what we must regard as the primitive 
form of vegetal circulation—simple absorption from cell to cell. These, 
when they have lost much of their water, will take up the dye to some dis- 
tance through their general substance, or rather through its interstices, even 
neglecting the vessels. At other times, in the same leaves, the vessels will 
become charged while comparatively little absorption takes place through 
the cellular tissue. Even in these exceptional cases, however, the movement 
through cellular tissue is nothing like as fast as the movement through 
vessels, 


556 


for sap will, other things being equal, cause currents towards the 
most rapidly-growing parts—towards unfolding shoots and leaves, 
but not towards adult leaves. Next we have evaporation, acting 
more on the adult leaves than on those which are in the bud, 
or but partially developed. This evaporation is both regularly 
and irregularly intermittent. Depending chiefly on the action 
of the sun, it is, in fine weather, greatly checked or wholly 
arrested every evening; and in cloudy weather must be much 
retarded during the day. Further, every hygrometric variation, 
as well as every variation in the movement of the air, must 
vary the evaporation. his chief action, therefore, which, by con- 
tinually emptying the ends of the capillary tubes, makes upward 
currents possible, is one which intermits every night, and every day 
is strong or feeble as circumstances determine. ‘Then, in the third 
place, we have this rude pumping process above described, going on 
with greater vigour when the wind is violent, and with less 
vigour when it is gentle — drawing liquid towards different 
parts according to their degrees of oscillation, and from diffe- 
rent parts according as they can most readily furnish it. And 
now let us ask what must result under changing conditions from 
these variously-conflicting and conspiring forces. When a warm 
sunshine, causing rapid evaporation, is emptying the vessels of the 
leaves, the osmotic and capillary actions that refill them will be 
continually aided by the pumping action of the swaying petioles, 
twigs, and branches, provided their oscillations are moderate. Under 
these conditions the current of sap, moving in the direction of least 
resistance, will set towards the leaves. But what will happen when 
the sun sets? There is now nothing to determine currents either 
upwards or downwards, except the relative rates of growth in the 
parts and the relative demands set up by the oscillations; and the 
oscillations acting alone, will draw sap to the oscillating parts as 
much from above as from below. If the resistance to be overcome 
by a current setting back from the leaves is less than the resistance 
to be overcome by a current setting up from the roots, then a 
current will set back from the leaves. Now it is, I think, tolerably 
manifest that in the swaying twigs and minor branches, less force 
will be required to overcome the inertia of the short columns of 
liquid between them and the leaves than to overcome the inertia of 
the long columns between them and the roots. Hence during the 
night, as also at other times when evaporation is not going on, the 
sap will be drawn out of the leaves into the adjacent supporting 
parts ; and their nutrition will be increased. If the wind is strong 
enough to produce a swaying of the thicker branches, the back 
current will extend to them also; and a further strengthening will 
result from their absorption of the elaborated sap. And when the 
great branches and the stem are bent backwards and forwards by a 


557 


gale, they too will share in the nutrition. It may at first sight seem 
that these parts, being nearer to the roots than to the leaves, will 
draw their supplies from the roots only. But the quantity which the 
roots can furnish is insufficient to meet so great a demand. Under 
the conditions described, the exudation of sap from the vessels will 
be very great, and the draught of liquid required to refill them, not 
satisfied by that which the root-fibres can take in, will extend to the 
leaves. Thus sap will flow to the several parts according to their 
respective degrees of activity—to the leaves while light and heat 
enable them to discharge their functions, and back to the twigs, 
branches, stem, and roots when these become active and the leaves 
inactive, or when their activity dominates over that of the leaves. 
And this distribution of nutriment, varying with the varying 
activities of the parts, is just such a distribution as we know must be 
required to keep up the organic balance. 

To this explanation it may be objected that it does not account 
for the downward current of sap in plants that are sheltered. The 
stem and roots of a drawing-room Geranium display a thickening 
which implies that nutritive matters have descended from the leaves, 
although there are none of those oscillations by which the sap is said 
to be drawn downwards as well as upwards. The reply is, that the 
stem and roots tend to repeat their typical structures, and that the 
absorption of sap for the formation of their respective dense tissues, 
is here the force which determines the descent. Indeed it must be 
borne in mind that the mechanical strains and the pumping process 
which they keep up, as well as the distention caused by osmose, do 
not in themselves produce a current either upwards or downwards: 
they simply help to move the sap towards that place where there is 
the most rapid abstraction of it—the place towards which its motion 
is least resisted. Whether there is oscillation or whether there is 
not, the physiological demands of the different parts of the plant 
determine the direction of the current ; and all which the oscillations 
and the distention do is to facilitate the supply of these demands. 
Just as much, therefore, in a plant at rest as in a plant -in motion, 
the current will set downwards when the function of the leaves is 
arrested, and when there is nothing to resist that abstraction of sap 
caused by the tendency of the stem- and root-tissues to assume their 
typical structures. To which admission, however, it must be added 
that since this typical structure assumed, though imperfectly assumed, 
by the hot-house plant, is itself interpretable as the inherited effect 
of external mechanical actions on its ancestors, we may still consider 
the current set up by the assumption of the typical structure to be 
indirectly due to such actions. 

Interesting evidence of another order here demands notice. In the 
course of experiments on the absorption of dyes by leaves, it 
happened that in making sections parallel to the plane of a leaf, with 


558 


the view of separating its middle layer containing the vessels, I came 
upon some structures that were new to me. ‘These structures, where 
they are present, form the terminations’of the vascular system. They 
are masses of irregular and imperfectly united fibrous cells, such as 
those out of which vessels are developed; and they are sometimes 
slender, sometimes bulky—usually, however, being more or less club- 
shaped. In transverse sections of leaves their distinctive characters 
are not shown: they are taken for the smaller veins. It is only by 
carefully slicing away the surface of a leaf until we come down 
to that part which contains them, that we get any idea of their 
nature. Fig. 1 represents a specimen taken from a leaf of 
Euphorbia neriifolia. Occupying one of the interspaces of the ulti- 
mate venous network, it consists of a spirally-lmed duct or set of 
ducts, which connects with the neighbouring vein a cluster of half- 
reticulated, half-scalariform cells. These cells have projections, many 
of them tapering, that insert themselves into the adjacent intercellular 
spaces, thus producing an extensive surface of contact between the 
organ and the imbedding tissues. A further trait is, that the en- 
sheathing prosenchyma is either but little developed or wholly ab- 
sent ; and consequently this expanded vascular structure, especially 
at its end, comes immediately in contact with the tissues concerned 
in assimilation. The leaf of Kuphorbia neriifolia is a very fleshy 
one; and in it these organs are distributed through a compact, 
though watery, cellular mass. But in any leaf of the ordinary type 
which possesses them, they lie in the network parenchyma composing 
its lower layer ; and wherever they occur in this layer its cells unite 
to enclose them. This arrangement is shown in fig. 2, representing 
a sample from the Caoutchouc-leaf, as seen with the upper part of 
its envelope removed; and it is shown still more clearly in a sample 
from the leaf of Panax Lessonii, fig. 8. Figures 4 and 5 represent, 
without their sheaths, other such organs from the leaves of Panax 
Lessonii and Clusia flava. Some relation seems to exist between 
their forms and the thicknesses of the layers in which they lie. 
Certain very thick leaves, such as those of Clusia flava, have them 
less abundantly distributed than is usual, but more massive. Where 
the parenchyma is developed not to so great an extreme, though 
still largely, as in the leaves of Holly, Aucuba, Camellia, they are 
not so bulky; and in thinner leaves, like those of Privet, Elder, 
&c., they become longer and less conspicuously club-shaped. Some 
adaptations to their respective positions seem implied by these modi- 
fications ; and we may naturally expect that in many thin leaves 
these free ends, becoming still narrower, lose the distinctive and 
suggestive characters possessed by those shown in the diagrams. 
Relations of this kind are not regular, however. In various other 
genera, members of which I have examined, as Rhus, Viburnum, 
Griselinia, Brexia, Botryodendron, Pereskia, the variations in the 


559 


bulk and form of these structures are not directly determined by 
the spaces which the leaves allow: obviously there are other modi- 
fying causes. It should be added that while these expanded free 
extremities graduate into tapering free extremities, not differing 
from ordinary vessels, they also pass insensibly into the ordinary in- 
osculations. Occasionally, along with numerous free endings, there 
occur loops ; and from such loops there are transitions to the ulti- 
mate meshes of the veins. 

These organs are by no means common to all leaves. In many 
that afford ample spaces for them they are not to be found. So far 
as I have observed, they are absent from the thick leaves of plants 
which form very little wood. In Sempervivum, in Echeveria, in 
Lryophyllum, they do not appear to exist; and I have been unable 
to discover them in Kalanchoé rotundifolia, in Kleinia ante-euphorbium 
and ficordes, in the several species of Crassula, and in other succulent 
plants. It may be added that they are not absolutely confined to 
leaves, but occur in stems that have assumed the functions of leaves. 
At least I have found, in the green parenchyma of Opuntia, organs 
that are analogous though much more rudely and irregularly formed. 
In other parts, too, that have usurped the leaf-function, they occur, 
as in the phyllodes of the Australian Acacias. These have them 
abundantly developed; and it is interesting to observe that here, 
where the two vertically-placed surfaces of the flattened-out petiole 
are equally adapted to the assimilative function, there exist two 
layers of these expanded vascular terminations, one applied to the 
inner surface of each layer of parenchyma. 

Considering the structures and positions of these organs, as well 
as the natures of the plants possessing them, may we not form a 
shrewd suspicion respecting their function? Is it not probable that 
they facilitate absorption of the juices carried back from the leaf for 
the nutrition of the stem and roots? They are admirably adapted 
for performing this office. Their component fibrous cells, having 
angles insinuated between the cells of the parenchyma, are shaped 
just as they should be for taking up its contents; and the absence 
of sheathing tissue between them and the parenchyma facilitates the 
passage of the elaborated liquids. Moreover there is the fact that 
they are allied to organs which obviously have absorbent functions. 
I am indebted to Dr. Hooker for pointing out the figures of two 
such organs in the “ Icones Anatomice” of Link. One of them is 
from the end of a dicotyledonous root-fibre, and the other is from 
the prothallus of a young Fern. In each case a cluster of fibrous 
cells, seated at a place from which liquid has to be drawn, is con- 
nected by vessels with the parts to which liquid has to be carried 
There can scarcely be a doubt, then, that in both cases absorption 
is effected through them. I have met with another such organ, 
more elaborately constructed, but evidently adapted to the same 


560 


office, in the common Turnip-root. As shown by the end view 
and longitudinal section in figs. 6 and 7, this organ consists of 
rings of fenestrated cells, arranged with varying degrees of regu- 
larity into a funnel, ordinarily having its apex directed towards the 
central mass of the Turnip, with which it has, in some cases at least, 
a traceable connexion by a canal. Presenting as it does an external 
porous surface terminating one of the branches of the vascular sys- 
tem, each of these organs is well fitted for taking up with rapidity 
the nutriment laid by in the Turnip-root, and used by the plant 
when it sends up its flower-stalk. Nor does even this exhaust the 
analogies. The cotyledons of the young bean, experimented upon 
as before described, furnished other examples of such structures, 
exactly in the places where, if they are absorbents, we might 
expect to find them. Amid the branchings and inosculations of the 
vascular layer running through the mass of nutriment deposited in 
each cotyledon, there are conspicuous free terminations that are club- 
shaped, and prove to be composed, like those in leaves, of irregularly 
formed and clustered fibrous cells; and some of them, diverging 
from the plane of the vascular layer, dip down into the mass of 
starch and albumen which the young plant has to utilize, and which 
these structures can have no other function but to take up. 

Besides being so well fitted for absorption, and besides being 
similar to organs which we cannot doubt are absorbents, these 
vascular terminations in leaves afford us yet another evidence of their 
functions. They are seated in a tissue so arranged as specially to 
facilitate the abstraction of liquid. The centripetal movement of the 
sap must be set up by a force that is comparatively feeble, since, the 
parietes of the ducts being porous, air will enter if the tension on the 
contained columns becomes considerable. Hence it is needful that 
the exit of sap from the leaves should meet with very little resistance. 
Now were it not for an adjustment presently to be described, it would 
meet with great resistance, notwithstanding the peculiar fitness of 
these organs to take it in. Liquid cannot be drawn out of any 
closed cavity without producing a collapse of the cavity’s sides; 
and if its sides are not readily collapsible, there must be a corre- 
sponding resistance to the abstraction of liquid from it. Clearly the 
like must happen if the liquid is to be drawn out of a tissue which 
cannot either diminish in bulk bodily or allow its components indi- 
vidually to diminish in bulk. In an ordinary leaf, the upper layer of 
parenchyma, formed as it is of closely-packed cells that are without 
interspaces, and are everywhere held fast within their framework of 
veins, can neither contract easily as a mass, nor allow ifs separate 
cells to do so. Quite otherwise is it with the network-parenchyma 
below. The long cells of this, united merely by their ends and 
having their flexible sides surrounded by air, may severally have 
their contents considerably increased and decreased without offering 


bot 


appreciable resistances; and the network-tissue which they form will, 
at the same time, be capable of undergoing slight expansions and con- 
tractions of its thickness. In this layer occur these organs that are so 
obviously fitted for absorption. Here we find them indirect communica- 
tion with its system of collapsible cells. The probability appears to be, 
that when the current sets into the leaf, it passes through the vessels 
and their sheaths chiefly into the upper layer of cells (this upper 
layer having a larger surface of contact with the veins than the 
lower layer, and being the seat of more active processes) ; and that 
the juices of the upper layer, enriched by the assimilated matters, 
pass into the network parenchyma, which serves as a reservoir from 
which they are from time to time drawn for the nutrition of the rest 
of the plant, when the actions determine the downward current. 
Should it be asked what happens where the absorbents, instead of 
being inserted in a network parenchyma, are, as in the leaves of 
Euphorbia neriifolia, inserted in a solid parenchyma, the reply is, 
that such a parenchyma, though not furnished with systematically 
arranged air-chambers, nevertheless contains air in its intercellular 
spaces; and that when there occurs a draught upon its contents, 
the expansion of this air and the entrance of more from without, 
quickiy supply the place of the abstracted liquid. 

If, then, returning to the general argument, we conclude that these 
expanded terminations of the vascular system in leaves are absorbent 
‘organs, we find a further confirmation of the views set forth respect- 
ing the alternating movement of the sap along the same channels. 
These spongioles of the leaves, like the spongioles of the roots, being 
appliances by which liquid is taken up to be carried into the mass of 
the plant. we are obliged to. regard the vessels that end in these 
spongioles of the leaves as being the channels of the down current 
‘whenever it is produced. If the elaborated sap is abstracted from 
the leaves by these absorbents, then we have no alternative but to 
suppose that, having entered the vascular system, the elaborated sap 
descends through it. And seeing how, by the help of these special 
terminations, it becomes possible for the same vessels to carry back 
a quality of sap unlike that which they bring up, we are enabled to 
understand tolerably well how this rhythmical movement produces a 
downward transfer of materials for growth. 


The several lines of argument may now be brought together; and 
along with them may be woven up such evidences as remain. Let 
me first point out the variety of questions to which the hypothesis 
‘supplies answers. 

It is required to account for the ascent of sap to a height beyond 
‘that to which capillary action can raise it. This ascent is accounted 
for by the propulsive action of transverse strains, joined with that of 
osmotic distention. A cause has to be assigned for that rise of sap 

VOL. I}. 36 


5€2 


which, in the spring, while yet there is no considerable evaporation 
to aid it, goes on with a power which capillarity does not explain, 
The co-operation of the same two agencies is assignable for this result 
also.* The circumstance that vessels and ducts here contain sap and 
there contain air, and at the same place contain at different seasons 
now air and now sap is a fact calling for explanation. An cxplana- 
tion is furnished by these mechanical actions which involve the en- 
trance or expulsion of air according to the supply of liquid. That 
vessels and ducts which were originally active sap-carriers go com- 
pletely out of use, and have their function discharged by other 
vessels or ducts, is an anomaly that has to be solved. Again, we 
are supplied with a solution: these deserted vessels and ducts are 
those which, by the formation of dense tissue outside of them, be- 
come so circumstanced that they cannot be compressed as they 
originally were. A channel has to be found for the downward 
current of sap, which, on any other hypothesis than the foregoing, 
must be a channel separate from that taken by the upward 
current; and yet no good evidence of a separate channel has been 
pointed out. Here, however, the difficulty disappears, since one 
channel suffices for the current alternating upwards and downwards 
according to the conditions. Moreover there has to be found a 
force producing or facilitating the downward current, capable even 
of drawing sap out of drooping branches; and no such force is 
forthcoming. The hypothesis set forth dispenses with this necessity : 
under the recurring change of conditions, the same distention and 
oscillation which before raised the sap to the places of consumption, 
now bring it down to the places of consumption. A physical 
process has to be pointed out by which the material that forms 
dense tissue is deposited at the places where it is wanted, rather 
than at other places. This physical process the hypothesis in- 
dicates. It is requisite to find an explanation of the fact that, 
when plants ordinarily swayed about by the wind are grown indoors, 
the formation of wood is so much diminished that they become abnor- 
mally slender. Of this an explanation is supplied. Yet a further 


* It seems probable, however, that osmotic distention is here, especially, 
the more important of the two factors. The rising of the sap in spring may 
indirectly result, like the sprouting of the seed, from the transformation of 
starch into sugar. During germination, this change of an oxy-hydro-carbon 
from an insoluble into a soluble form, leads to rapid endosmose ; con- 
sequently to great distention of the seed ; and therefore to a force which 
thrusts the contained liquids into the plumule and radicle, and gives them 
power to displace the soil in their way : it sets up an active internal move- 
ment when neither evaporation nor the change which light produces can be 
operative. And similarly, if, in the spring, the starch stored up in the 
roots of a tree passes into the form of sugar, the unusual osmotic absorption 
that arises will cause an unusual distention—a distention which, being 
resisted by the tough bark of the roots and stem, will result in a powerful 
upward thrust of the contained liquid. 


563 


fact to be interpreted is, that in the same individual plant homologous 
parts, which, according to the type of the plant, should be equally 
woody, become much thicker one than another if subject to 
greater mechanical stress. And of this too an interpretation is 
similarly afforded. 

Now the sufficiency of the assigned actions to account for so many 
phenomena not otherwise explained, would be strong evidence that 
the rationale is the true one, even were it of a purely hypothetical 
kind. How strong, then, hecomes the reason for believing it the 
true one when we remember that the actions alleged demonstrably go 
on in the way asserted. They are ever operating before our eyes; 
and that they produce the effects in question is a conclusion dedu- 
cible from mechanical principles, a conclusion established by induction, 
and a conclusion verified by experiment. These three orders of 
proof may be briefly summed up as follows. 

That plants which have to raise themselves above the earth’s sur- 
face, and to withstand the actions of the wind, must have a power of 
developing supporting structure, is an @ prior? conclusion which may 
be safely drawn. It is an equally safe @ priori conclusion, that if 
the supporting structure, either as a whole or in any of its parts, has 
to adapt itself to the particular strains which the individual plant is 
subject to by its particular circumstances, there must be at work 
some process by which the strength of the supporting structure is 
everywhere brought into equilibrium with the forces it has to bear. 
Though the typical distribution of supporting structure in each kind 
of plant may be explained teleologically by those whom teleological 
explanations satisfy ; and though otherwise this typical distribution 
may be ascribed to natural selection acting apart from any directly 
adaptive process ; yet it is manifest that those departures from the 
typical distribution which fit the parts of each plant to their special 
conditions are explicable neither teleologically nor by natural selec- 
tion. We are, therefore, compelled to admit that, if in each plant 
there goes on a balancing of the particular strains by the particular 
strengths, there must be a physical or physico-chemical process by 
which the adjustments of the two are effected. Meanwhile we are 
equally compelled to admit, a priori, that the mechanical actions to be 
resisted, themselves affect the internal tissues in such ways as to fur- 
ther the increase of that dense substance by which they are resisted. 
It is demonstrable that bending the petioles, shoots, and stems must 
compress the vessels beneath their surfaces, and increase the exuda- 
tion of nutritive matters from them, and must do this actively in pro- 
portion as the bends are great and frequent; so that while, on the 
one hand, it is a necessary deduction that, if the parts of each plant 
are to be severally strengthened according to the several strains, 
there must be some direct connexion between strains and strengths, 
it is, on the other hand, a necessary deduction from mechanical prin- 

36 * 


564 


ciples that the strains do act in such ways as to aid the increase of 
the strengths. How a like correspondence between two @ priori 
arguments holds in the case of the circulation, needs not to be shown 
in detail. It will suffice to remind the reader that while the 
raising of sap to heights beyond the limit of capillarity implies some 
force to effect it, we have in the osmotic distention and the intermit- 
tent compressions caused by transverse strains, forces which, under 
the conditions, cannot but tend to effect it-; and similarly with the re- 
quirement for a downward current, and the production of a down- 
ward current. 

Among the inductive proofs we find a kindred agreement. Diffe- 
rent individuals of the same species, and different parts of the same 
individual, do strengthen in different degrees; and there is a clearly 
traceable connexion between their strengthenings and the intermittent 
strains they are exposed to. This evidence, derived from contrasts 
between growths on the same plant or on plants of the same type, is 
enforced by evidence derived from contrasts between plants of diffe- 
rent types. The deficiency of woody tissue which we see in plants 
called succulent, is accompanied by a bulkiness of the parts which 
prevents any considerable oscillations ; and this character is also habi- 
tually accompanied by a dwarfed growth. When, leaving these rela- 
tions as displayed externally, we examine them internally, we find 
the facts uniting to show, by their agreements and differences, that 
between the compression of the sap-canals and the production of 
wood there is a direct relation. We have the facts, that in each 
plant, and in every new part of each plant, the formation of sap- 
canals precedes the formation of wood; that the deposit of woody 
matter, when it begins, takes place around these sap-canals, and 
afterwards around the new sap-canals successively developed; that 
this formation of wood around the sap-canals takes place where the 
coats of the canals are demonstrably permeable, and that the amount 
of wood-formation is proportionate to the permeability. And then 
that the permeability and extravasation of sap occur wherever, in 
the individual or in the type, there are intermittent compressions, is 
proved alike by ordinary cases and by exceptional cases. In the 
one class of cases we see that the deposit of wood round the vessels 
begins to take place when they come into positions that subject 
them to intermittent compressions, while it ceases when they become 
shielded from compressions. And in the other class of cases, where, 
from the beginning, the vessels are shielded from compression by sur- 
rounding fleshy tissue, there is a permanent absence of wood-forma- 
tion. 

To which complete agreement between the deductive and induce 
tive inferences has to be added the direct proof supplied by experi- 
ments. It is put beyond doubt by experiment that the liquids ab- 
sorbed by plants are distributed to their different parts through their 


565 | 


vessels—at first. by the spiral or allied vessels originally developed, 
and then by the better-placed ducts formed later, By experiment - 
it is demonstrated that the intermittent compressions caused by os- , 
cillations urge the sap along the vessels and ducts, And it is also ex- _ 
perimentally proved that the same intermittent compressions produce 
exudation of sap from vessels and ducts into the surrounding tissue. 
That the processes here described, acting through all past time, 
have sufficed of themselves to develope the supporting and distribut- 
ing structures of plants, is not alleged. What share the natural 
selection of variations distinguished as spontaneous, has had in estab- 
lishing them, is a question.-which remains to be discussed. Whether 
acting alone natural selection would have sufficed to evolve these 
vascular and resisting tissues, I do not profess to say. That it has 
been a co-operating cause, I take to be self-evident : it must all along 
have furthered the action of any other cause, by preserving the in- 
dividuals on which such other cause had acted most favourably. 
Seeing, however, the conclusive proof which we have that another 
cause has been in action—certainly on individuals, and, in all proba- 
bility, by inheritance on races—we may most philosophically ascribe 
the genesis of these internal structures to this cause, and regard 
natural selection as having here played the part of an accelerator. 


EXPLANATION OF PLATE, 

Fig. 1. Absorbent organ from the leaf of Huphorbia neritfolia. 
The cluster of fibrous cells forming one of. the terminations of the 
vascular system is here imbedded in a solid parenchyma. 

Fig. 2. A structure of analogous kind irom the leaf. of Ficus 
elastica. Here the expanded terminations of the vessels are im- 
bedded in the network parenchyma, the cells of which unite to form 
envelopes for them. 

Fig. 3.. Shows on a larger scale one of these absorbents from 
the leaf of Panax Lessonii. In this figure is clearly seen the way in 
which the cells of the network parenchyma unite into a SON fitting 
case for the spiral cells. 

Fig. 4. Represents a m uch more massive absorbent from the same 
leaf, the surrounding tissues being omitted. 

Fig. 5. Similarly represents, without its sheath, an absorbent from 
the leaf of Clusia flava. 

Fig. 6. End view of an absorbent organ from the root of a 
Turnip. It is taken from the outermost layer of vessels. Its funnel- 
shaped interior is drawn as it presents itself when looked at from the 
outside of this layer, its narrow end being directed towards the 
centre of the Turnip. 

Fig. 7 A longitudinal section through the axis of another suede 
organ, showing its annuli of reticulated cells when cut through. The 
cellular tissue which fills the interior is supposed to be removed 


566 


Fig. 8. A less-developed absorbent, showing its approximate con- 
nexion with a duct. In their simplest forms, these structures consist 
of only two fenestrated cells, with their ends bent»round so as to 
meet. Such types occur in the central mass of the Turnip, where 


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the vascular system is relatively imperfect. Besides the compara- 
tively regular forms of these absorbents, there are forms composed 
of amorphous masses of fenestrated cells. It should be added that 
both the regular and irregular kinds are very variable in their num- 
bers : in some turnips they are abundant, and in others scarcely to be 
found. Possibly their presence depends on the age of the Turnip. 


AL bb ea XL, 


ON THE ORIGIN OF THE VERTEBRATE TYPE. 


[When studying the development of the vertebrate skeleton, there 
occurred to me the following idea respecting the possible origin of the 
notochord. 1 was eventually led to omit the few pages of Appendix in 
which I had expressed this idea, because it was unsupported by develop- 
mental evidence. The developmental evidence recently discovered, how- 
ever, has led Professor Haeckel and others to analogous views respecting 
the affiliation of the Vertebrata on the Molluscoida. Having fortu- 
nately preserved a proof of the suppressed pages, I am able now to 
add them. With the omission of a superfluous paragraph, they are 
reprinted verbatim from this proof, which dates back to the autumn of 
1865, at which time the chapter on “The Shapes of Vertebrate 
Skeletons” was written.—December, 1869. | 


The general argument contained in Chap..X VI. of Part IV., I 
have thought it undesirable to implicate with any conception more 
speculative than those essential to it; and to avoid so implicating 
it, I transfer to this place an hypothesis respecting the derivation 
of the rudimentary vertebrate structure, which appears to me 
worth considering. 

Among those molluscoid: animals with which the lowest verte- 
brate animal has-sundry: traits in common, it very generally happens 
that while the adult is stationary the larva is locomotive. The 
locomotion of the larva is effected by the undulations of a tail. In 
shape and movement one of these young Ascidians is not altogether 
unlike a Tadpole. And as the tail of the Tadpole disappears 
when its function comes to be fulfilled by limbs; so the Ascidian 
larva’s tail disappears when fixation of the larva renders it useless. 
This disappearance of the tail, however, is not without exception. 
The Appendicularia is an Ascidian which retains its tail through- 
out life; and by its aid continues throughout life to swim about. 
Now this tail of the Appendicularia has a very suggestive structure. 
It is long, tapering to a point, and flattened. Irom end to end: 
there runs a mid-rib, which appears to be an imbedded gelatinous 
rod, not unlike a notochord. Extending along the two sides of. 


568 APPENDIX. 


this mid-rib, are bundles of muscular fibres; and its top bears a 
gangliated nervous thread, giving off, at intervals, branches to the 
muscular fibres. In the Appendicularia this tail, which is inserted 
at the lower part of the back, is bent forwards, so as not to be 
adapted for propelling the body of the animal head foremost ; but 
the homologous tails of the larval Ascidians are directed backwards, 
so as to produce forward movement. If we suppose a type like the 
Appendicularia in the structure and insertion of its permanent tail, 
but resembling the larval forms in the direction of its tail, it is, I 
think, not difficult to see that functional adaptation joined with 
natural selection, might readily produce a type approximating to 
that whose origin we are considering. It is a fair assumption 
that an habitually - locomotive creature would profit by in- 
creased power of locomotion. This granted, it follows that 
such further development of the tail-structures as might arise 
from enhanced function, and such better distribution of them 
as spontaneous variation might from time to time initiate, 
would be perpetuated. What must be the accompanying changes? 
The more vigorous action of such an appendage implies a firmer 
insertion into the body; and this would be effected by the pro- 
longation forwards of the central axis of the tail into the creature’s 
back. As fast as there progressed this fusion of the increasingly- 
powerful tail with the body, the body would begin to partake of its 
oscillations ; and at the same time that the resistant axis of the tail 
advanced along the dorsal region, its accompanying muscular fibres 
would spread over the sides of the body: gradually taking such. 
modified directions and insertions as their new conditions rendered 
most advantageous. Without further explanation, those who 
examine drawings of the structures described, will, I think, 
see that in such a way a tail homologous with that of the 
Appendicularia, would be likely, in the course of that de- 
velopment required for its greater efficiency, gradually to 
encroach on the body, until its mid-rib became the dorsal 
axis, its gangliated nerve-thread the spinal chord, and _ its 
muscular fibres the myocommata. Such a development of an 
appendage into a dominant part of the organism, though at first 
sight a startling supposition, is not without plenty of parallels: 
instance the way in which the cerebral ganglia, originally mere 
adjuncts of the spinal chord, eventually become the great centres of 
the nervous system to which the spinal chord is quite subordinate ; 
or instance the way in which the limbs, small and inconspicuous in 
fishes, become, in Man, masses which, taken together, outweigh the 
trunk. It may be added that these familiar cases have a further 
appropriateness ; for thvy exhibit higher degrees of that same 
increasing dominance of the organs of external relation, which the 
hypothesis itself implies. 


APPENDIX. 569 


Of course, if the rudimentary vertebrate apparatus thus grew 
into, and spread over, a molluscoid visceral system, the formation 
of the notochord under the action of alternating transverse strains, 
did not take place as suggested in § 255; but it does not therefore 
follow that its differentiation from surrounding tissues was ‘not 
mechanically initiated in the way described. For what was said in 
that section respecting the effects of lateral bendings of the body, 
equally applies to lateral bendings of the tail; and as fast as the 
developing tail encroached on the body, the body would become 
implicated in the transverse strains, and the differentiation would 
advance forwards under the influences originally alleged. Obviously, 
too, though the lateral muscular masses would in this case have a 
different history ; yet the segmentation of them would be eventually 
determined by the assigned causes. For as fast as the strata of 
contractile fibres, developing somewhat in advance of the dorsal 
axis, spread along the sides, they would come under the influence 
of the alternate flexions ; and while, by survival of the fittest, their 
parts became adjusted in direction, their segmentation would, as 
before, accompany their increasing massiveness. The actions and 
reactions due to lateral undulations would still, therefore, be the 
causes of differentiation, with which natural seleetion would co- 
operate. 


é 


“ule 


APPHNDIX H. 


THE SHAPES AND ARRANGEMENTS OF FLOWERS, 


In Part IV., Chapter X., under the title of ‘‘ The Shapes of 
Flowers,” I have, after describing their several kinds of symmetry, 
as habitually related to their positions, made some remarks by way 
of interpretation. The truth that flowers exhibit a radial symmetry 
when they are so placed as to be equally affected all round by 
incident forces, having been exemplified, and also the truth that 
they assume a bilateral symmetry when they are so placed that 
their two sides are conditioned in ways different from the ways in 
which their upper and lower parts are conditioned; I have gone 
on to inquire (in § 234) by what causes such modifications of 
form are produced. I have stated that, originally, I inclined to 
ascribe them entirely to differences in the relations of the parts 
to physical forces—light, heat, gravitation, etc. ; but that I found 
sundry facts stood in the way of this interpretation. And I have 
said that “ Mr. Darwin’s investigations into the fertilization of 
Orchids led me to take into account an unnoticed agency.” Con- 
tinuing to recognize the physical forces as factors having some 
influence, I have concluded that the most important factor is the 
action of insects; which, aiding most the fertilization of those 
flowers which most facilitate their entrance, produce, in course of 
generations, a form of flower specially adapted to the special 

osition. 

Though still adhering to this interpretation, I have since found 
reason to think that the original interpretation contains a larger 
portion of truth than I supposed at the time when I was led thus to 
revise it. While staying at Murren, in Switzerland, in 1872, I ob- 
served some modifications in a species of Gentian, which proved to me 
thatthe action of incident physical forces on flowers is, in some cases, 
very rapid and decided. The species furnishing this evidence was the 
Gentiana Asclepiadea; which I found in a copse formed of bushes that 
were here wide apart and there close together. In some places not 
near to the bushes, the individuals of the species grew vertically ; in 
other places, partially shaded, their inclined shoots curved in such 


672 


directions as to get the most light; and in other cases their shoots 
were led to take directions almost or quite horizontal. That, along 
with these modifications in the directions of their shoots, there went 
adjustments in the attitudes of their leaves, was a fact not specially 
worthy of remark ; for plants placed inside the windows of houses 
habitually show us that leaves quickly bend themselves into atti- 
tudes giving them the greatest amounts of light. But the fact 
which attracted my attention was, that the flowers changed their 
attitudes in an equally-marked manner. The radial distribution 
passed into a bilateral distribution with the greatest readiness. 
Comparison of the annexed figures will show the character of this 
change. 

Figure I. represents part of a vertically growing shoot. This 
belonged to an individual growing unimpeded by bushes, and getting 
light on all sides. Here it is observable that the pairs of leaves, 
placed alternately in directions transverse to one another—one pair 
pointing, say, north and south, and the next pair pointing east and 
west—maintain, taking them in the aggregate, a radial distribution ; 
and it is also observable that the alternate pairs of flowers are 
similarly arranged. 

Figure II. is a sketch from a shoot which leaned towards one 
side, and of which the higher part, as it bent more and more, 
got its upper side more and more differently conditioned from its 
lower side. Here we find that not only the leaves, but also the 
flowers, have adjusted themselves to the changed conditions. The 
leaves of the lowest pair hang out in the normal way, on the op: 
posite sides of the axis, so that a plane passing through their sur- 
faces will cut the axis transversely; and their two axillary flower- 
buds, c and d, are similarly placed on opposite sides of the axis. 
But at the other part of the shoot, we see both that the leaves have 
adjusted themselves so that their planes, no longer cutting the axis 
transversely, keep a fit adjustment with respect to the light; and 
also that the flowers, no longer on opposite sides of the axis, have 
bent round to the upper side, as at a and 0. 

Figure III. shows us this re-arrangement carried: still further. 
The shoot it represents was growing in a direction nearly horizontal, 
and therefore receiving the light only on one side. And here, 
besides seeing that the leaves have so adjusted themselves that they. 
all lie in approximately the same plane, which is parallel to the axis 
instead of transverse to it, we see that the two pairs of flower-buds 
have both come round to the upper side of the axis. So: that 
in this shoot, the original radial symmetry in the arrangement 
of leaves and flowers, is completely changed inte a bilateral 
symmetry. ats 

These facts do not, it is true, prove any modification in the forms 


574 


of the flowers themselves: they only prove modification in the 
grouping of the flowers. But beyond showing, as they do conclu- 
sively, how readily a bilateral arrangement of flowers is producible 
out of an arrangement that was not bilateral, by the action of light, 
etc. ; they give increased probability to the belief that changes in the 
shapes of flowers are producible by the same agencies. Doubtless 
this change in the attitudes of the flower-buds is due to the uction of 
light on their calyces and peduncles more than to its action on their 
unfolding corollas. But along with an action so decided on the growth 
of these sheathing and supporting organs containing chlorophy], it is 
scarcely probable that there is no action on the growth of the petals, 
containing other colouring matter; considering that in both cases the 
development of the colouring matter depends on the action of 
light, and considering also the effect of light on petals, familiarly 
shown by their opening and closing. And if even but a small 
effect is producible on the growth of the corolla, then it is to be 
expected that light will be an agent in changing the form of the 
corolla, when the attitude of the flower causes its parts to be dif- 
ferently exposed. For a small effect on the individual flower will 
become a great effect in the flowers of remote descendants; pro- 
vided the changed attitudes of the flowers preserve considerable 
constancy throughout the succession of individuals. 

Be this as it may, however, the facts I have here described, 
which I doubt not other observers have seen paralleled in other 
plants, are instructive, as showing how quickly certain metamor- 
phoses are produced, and as implying the easy establishment of such 
metamorphoses as permanent characters in a species, if the modify- 
ing condititions become permanent. ‘'The changes of arrangement 
I have pointed out, do not become permanent in this species because 
its individuals are variously affected by the modifying forces: on 
some they do not act at all, on some a little, on some much; and 
even on the same individual the different shoots are quite differently 
affected. But if the habit of this plant were greatly changed— 
if, for instance, by spreading into habitats yielding abundant nutri- 
ment, the plant became very luxuriant, and, multiplying its branches, 
grew shrub-like ; it is clear that, being shaded by one another, these 
branches would be habitually circumstanced in a way like that which 
we here see produces bilateralness in the distribution of the flowers, 
if not in the flowers themselves; and being thus permanently 
affected, would become permanently bilateral. Accumulating by 
inheritance, what is here only an individual peculiarity, would he- 
- come a peculiarity of the species—a specific character. 


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